--- a/dom/ipc/ContentParent.cpp
+++ b/dom/ipc/ContentParent.cpp
@@ -274,16 +274,17 @@
 #ifdef MOZ_WIDGET_ANDROID
 #  include "AndroidBridge.h"
 #  include "mozilla/java/GeckoProcessManagerWrappers.h"
 #  include "mozilla/java/GeckoProcessTypeWrappers.h"
 #endif
 
 #ifdef MOZ_WIDGET_GTK
 #  include <gdk/gdk.h>
+#  include "mozilla/WidgetUtilsGtk.h"
 #endif
 
 #include "mozilla/RemoteSpellCheckEngineParent.h"
 
 #include "Crypto.h"
 
 #ifdef MOZ_WEBSPEECH
 #  include "mozilla/dom/SpeechSynthesisParent.h"
@@ -2540,16 +2541,25 @@ bool ContentParent::BeginSubprocessLaunc
     mSubprocess->DisableOSActivityMode();
   }
 #endif
 
   nsCString parentBuildID(mozilla::PlatformBuildID());
   extraArgs.push_back("-parentBuildID");
   extraArgs.push_back(parentBuildID.get());
 
+#ifdef MOZ_WIDGET_GTK
+  // This is X11-only pending a solution for WebGL in Wayland mode.
+  if (StaticPrefs::dom_ipc_avoid_gtk() &&
+      StaticPrefs::widget_non_native_theme_enabled() &&
+      widget::GdkIsX11Display()) {
+    mSubprocess->SetEnv("MOZ_HEADLESS", "1");
+  }
+#endif
+
   // See also ActorDealloc.
   mSelfRef = this;
   mLaunchYieldTS = TimeStamp::Now();
   return mSubprocess->AsyncLaunch(std::move(extraArgs));
 }
 
 void ContentParent::LaunchSubprocessReject() {
   NS_ERROR("failed to launch child in the parent");

--- a/dom/ipc/tests/browser.ini
+++ b/dom/ipc/tests/browser.ini
@@ -32,8 +32,11 @@ skip-if = !e10s
 support-files = file_dummy.html
 skip-if = !e10s
 [browser_bug1686194.js]
 support-files = file_dummy.html
 [browser_gc_schedule.js]
 skip-if =
   verify
   fission && os == "linux" && asan  # Bug 1713905 - new Fission platform triage
+[browser_very_fission.js]
+support-files = file_dummy.html
+run-if = widget == "gtk"

new file mode 100644
--- /dev/null
+++ b/dom/ipc/tests/browser_very_fission.js
@@ -0,0 +1,38 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+"use strict";
+
+// This test creates a large number of content processes as a
+// regression test for bug 1635451.
+
+const TEST_PAGE =
+  "http://mochi.test:8888/browser/dom/ipc/tests/file_dummy.html";
+
+const NUM_TABS = 256;
+
+add_task(async () => {
+  let promises = [];
+  for (let i = 0; i < NUM_TABS; ++i) {
+    promises.push(
+      BrowserTestUtils.openNewForegroundTab({
+        gBrowser,
+        opening: TEST_PAGE,
+        waitForLoad: true,
+        forceNewProcess: true,
+      })
+    );
+  }
+
+  let tabs = [];
+  for (const p of promises) {
+    tabs.push(await p);
+  }
+
+  ok(true, "All of the tabs loaded");
+
+  for (const t of tabs) {
+    BrowserTestUtils.removeTab(t);
+  }
+});

--- a/gfx/gl/GLContextProviderWayland.cpp
+++ b/gfx/gl/GLContextProviderWayland.cpp
@@ -12,49 +12,59 @@
 namespace mozilla::gl {
 
 using namespace mozilla::gfx;
 using namespace mozilla::widget;
 
 static class GLContextProviderX11 sGLContextProviderX11;
 static class GLContextProviderEGL sGLContextProviderEGL;
 
+// Note that if there is no GTK display, `GdkIsX11Display` and
+// `GdkIsWaylandDisplay` will both return false.  That case can
+// currently happen only in X11 mode if the pref `dom.ipc.avoid-gtk`
+// is set (and applicable to this process).  Thus, these conditionals
+// check for the presence of Wayland rather than the absence of X11.
+//
+// In the future we'll want `dom.ipc.avoid-gtk` to also apply to
+// Wayland; at that time we'll need another way to communicate the
+// choice of window system.
+
 already_AddRefed<GLContext> GLContextProviderWayland::CreateForCompositorWidget(
     CompositorWidget* aCompositorWidget, bool aHardwareWebRender,
     bool aForceAccelerated) {
-  if (GdkIsX11Display()) {
-    return sGLContextProviderX11.CreateForCompositorWidget(
+  if (GdkIsWaylandDisplay()) {
+    return sGLContextProviderEGL.CreateForCompositorWidget(
         aCompositorWidget, aHardwareWebRender, aForceAccelerated);
   } else {
-    return sGLContextProviderEGL.CreateForCompositorWidget(
+    return sGLContextProviderX11.CreateForCompositorWidget(
         aCompositorWidget, aHardwareWebRender, aForceAccelerated);
   }
 }
 
 /*static*/
 already_AddRefed<GLContext> GLContextProviderWayland::CreateHeadless(
     const GLContextCreateDesc& desc, nsACString* const out_failureId) {
-  if (GdkIsX11Display()) {
+  if (GdkIsWaylandDisplay()) {
+    return sGLContextProviderEGL.CreateHeadless(desc, out_failureId);
+  } else {
     return sGLContextProviderX11.CreateHeadless(desc, out_failureId);
-  } else {
-    return sGLContextProviderEGL.CreateHeadless(desc, out_failureId);
   }
 }
 
 /*static*/
 GLContext* GLContextProviderWayland::GetGlobalContext() {
-  if (GdkIsX11Display()) {
+  if (GdkIsWaylandDisplay()) {
+    return sGLContextProviderEGL.GetGlobalContext();
+  } else {
     return sGLContextProviderX11.GetGlobalContext();
-  } else {
-    return sGLContextProviderEGL.GetGlobalContext();
   }
 }
 
 /*static*/
 void GLContextProviderWayland::Shutdown() {
-  if (GdkIsX11Display()) {
+  if (GdkIsWaylandDisplay()) {
+    sGLContextProviderEGL.Shutdown();
+  } else {
     sGLContextProviderX11.Shutdown();
-  } else {
-    sGLContextProviderEGL.Shutdown();
   }
 }
 
 }  // namespace mozilla::gl

--- a/image/decoders/icon/gtk/nsIconChannel.cpp
+++ b/image/decoders/icon/gtk/nsIconChannel.cpp
@@ -10,18 +10,16 @@
 
 #include "mozilla/DebugOnly.h"
 #include "mozilla/EndianUtils.h"
 #include "mozilla/NullPrincipal.h"
 #include "mozilla/CheckedInt.h"
 #include "mozilla/dom/ContentChild.h"
 #include "mozilla/gfx/Swizzle.h"
 #include "mozilla/ipc/ByteBuf.h"
-#include "mozilla/Preferences.h"
-#include "mozilla/StaticPrefs_dom.h"
 #include <algorithm>
 
 #include <gio/gio.h>
 
 #include <gtk/gtk.h>
 
 #include "nsMimeTypes.h"
 #include "nsIMIMEService.h"
@@ -402,18 +400,17 @@ nsresult nsIconChannel::GetIcon(nsIURI* 
   return rv;
 }
 
 nsresult nsIconChannel::Init(nsIURI* aURI) {
   nsresult rv;
   nsCOMPtr<nsIInputStream> stream;
 
   using ContentChild = mozilla::dom::ContentChild;
-  auto* contentChild = ContentChild::GetSingleton();
-  if (contentChild && mozilla::StaticPrefs::dom_ipc_remote_mozIcon()) {
+  if (auto* contentChild = ContentChild::GetSingleton()) {
     // Get the icon via IPC and translate the promise of a ByteBuf
     // into an actually-existing channel.
     RefPtr<ContentChild::GetSystemIconPromise> icon =
         contentChild->SendGetSystemIcon(aURI);
     if (!icon) {
       return NS_ERROR_UNEXPECTED;
     }
 

--- a/image/test/browser/browser_mozicon_file.js
+++ b/image/test/browser/browser_mozicon_file.js
@@ -1,24 +1,12 @@
 /* This Source Code Form is subject to the terms of the Mozilla Public
  * License, v. 2.0. If a copy of the MPL was not distributed with this
  * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
 
 "use strict";
 
 add_task(async function test_mozicon_file_no_sandbox() {
   assertFileProcess();
-  assertMozIconIsRemote();
   await createMozIconInFile("txt");
   await createMozIconInFile("exe");
   await createMozIconInFile("non-existent-bidule");
 });
-
-add_task(async function test_mozicon_file_no_sandbox_no_remote() {
-  await SpecialPowers.pushPrefEnv({
-    set: [["dom.ipc.remote-mozIcon", false]],
-  });
-  assertFileProcess();
-  assertMozIconIsNotRemote();
-  await createMozIconInFile("txt");
-  await createMozIconInFile("exe");
-  await createMozIconInFile("non-existent-bidule");
-});

--- a/image/test/browser/browser_mozicon_file_sandbox_headless.js
+++ b/image/test/browser/browser_mozicon_file_sandbox_headless.js
@@ -6,22 +6,8 @@
 
 add_task(async function test_mozicon_file_with_sandbox() {
   assertFileProcess();
   assertSandboxHeadless();
   await createMozIconInFile("txt");
   await createMozIconInFile("exe");
   await createMozIconInFile("non-existent-bidule");
 });
-
-// https://bugzilla.mozilla.org/show_bug.cgi?id=1695381#c0
-// with sandbox and no remote enabled, this is expected to fail
-add_task(async function test_mozicon_file_with_sandbox_no_remote() {
-  await SpecialPowers.pushPrefEnv({
-    set: [["dom.ipc.remote-mozIcon", false]],
-  });
-  assertFileProcess();
-  assertSandboxHeadless();
-  assertMozIconIsNotRemote();
-  await createMozIconInFile("txt", false);
-  await createMozIconInFile("exe", false);
-  await createMozIconInFile("non-existent-bidule", false);
-});

--- a/image/test/browser/browser_sandbox_headless.ini
+++ b/image/test/browser/browser_sandbox_headless.ini
@@ -1,12 +1,8 @@
 [DEFAULT]
 support-files =
   head.js
 prefs =
   security.sandbox.content.headless=true
-skip-if =
-  (os != 'linux') # the pref is only used on linux
-  tsan # timeout on test_mozicon_file_with_sandbox_no_remote
-  asan # timeout on test_mozicon_file_with_sandbox_no_remote
-  ccov # timeout on test_mozicon_file_with_sandbox_no_remote
+skip-if = (os != 'linux') # the pref is only used on linux
 
 [browser_mozicon_file_sandbox_headless.js]

--- a/image/test/browser/head.js
+++ b/image/test/browser/head.js
@@ -37,24 +37,16 @@ function assertFileProcess() {
   // Ensure that the file content process is enabled.
   assertPrefVal("browser.tabs.remote.separateFileUriProcess", true);
 }
 
 function assertSandboxHeadless() {
   assertPrefVal("security.sandbox.content.headless", true);
 }
 
-function assertMozIconIsRemote() {
-  assertPrefVal("dom.ipc.remote-mozIcon", true);
-}
-
-function assertMozIconIsNotRemote() {
-  assertPrefVal("dom.ipc.remote-mozIcon", false);
-}
-
 function getPage() {
   let filePage = undefined;
   const { Services } = ChromeUtils.import(
     "resource://gre/modules/Services.jsm"
   );
   switch (Services.appinfo.OS) {
     case "WINNT":
       filePage = "file:///C:/";

--- a/ipc/glue/GeckoChildProcessHost.cpp
+++ b/ipc/glue/GeckoChildProcessHost.cpp
@@ -598,16 +598,23 @@ class AutoCFTypeObject {
 
 // We start the unique IDs at 1 so that 0 can be used to mean that
 // a component has no unique ID assigned to it.
 uint32_t GeckoChildProcessHost::sNextUniqueID = 1;
 
 /* static */
 uint32_t GeckoChildProcessHost::GetUniqueID() { return sNextUniqueID++; }
 
+/* static */
+void GeckoChildProcessHost::SetEnv(const char* aKey, const char* aValue) {
+  MOZ_ASSERT(mLaunchOptions);
+  mLaunchOptions->env_map[ENVIRONMENT_STRING(aKey)] =
+      ENVIRONMENT_STRING(aValue);
+}
+
 void GeckoChildProcessHost::PrepareLaunch() {
   if (CrashReporter::GetEnabled()) {
     CrashReporter::OOPInit();
   }
 
 #if defined(XP_LINUX) && defined(MOZ_SANDBOX)
   SandboxLaunchPrepare(mProcessType, mLaunchOptions.get());
 #endif

--- a/ipc/glue/GeckoChildProcessHost.h
+++ b/ipc/glue/GeckoChildProcessHost.h
@@ -79,16 +79,20 @@ class GeckoChildProcessHost : public Chi
   // otherwise, it could happen immediately.
   //
   // GeckoChildProcessHost instances must not be deleted except
   // through this method.
   void Destroy();
 
   static uint32_t GetUniqueID();
 
+  // Call this before launching to set an environment variable for the
+  // child process.  The arguments must be UTF-8.
+  void SetEnv(const char* aKey, const char* aValue);
+
   // Does not block.  The IPC channel may not be initialized yet, and
   // the child process may or may not have been created when this
   // method returns.
   bool AsyncLaunch(StringVector aExtraOpts = StringVector());
 
   virtual bool WaitUntilConnected(int32_t aTimeoutMs = 0);
 
   // Block until the IPC channel for our subprocess is initialized and

--- a/modules/libpref/init/StaticPrefList.yaml
+++ b/modules/libpref/init/StaticPrefList.yaml
@@ -2256,29 +2256,38 @@
 
 # How often to check for CPOW timeouts (ms). CPOWs are only timed
 # out by the hang monitor.
 - name: dom.ipc.cpow.timeout
   type: uint32_t
   value: 500
   mirror: always
 
-# Temporary pref to allow disabling remoting of MozIcon
-- name: dom.ipc.remote-mozIcon
-  type: bool
-  value: true
-  mirror: always
-
 #ifdef MOZ_ENABLE_FORKSERVER
 - name: dom.ipc.forkserver.enable
   type: bool
   value: false
   mirror: once
 #endif
 
+#ifdef MOZ_WIDGET_GTK
+#
+# Avoid the use of GTK in content processes if possible, by running
+# them in headless mode, to conserve resources (e.g., connections to
+# the X server).  See the usage in `ContentParent.cpp` for the full
+# definition of "if possible".
+#
+# This does not affect sandbox policies; content processes may still
+# dynamically connect to the display server for, e.g., WebGL.
+- name: dom.ipc.avoid-gtk
+  type: bool
+  value: true
+  mirror: always
+#endif
+
 # Whether or not to collect a paired minidump when force-killing a
 # content process.
 - name: dom.ipc.tabs.createKillHardCrashReports
   type: bool
   value: @IS_NOT_RELEASE_OR_BETA@
   mirror: once
 
 # Allow Flash async drawing mode in 64-bit release builds.

--- a/testing/mochitest/tests/Harness_sanity/test_sanityEventUtils.html
+++ b/testing/mochitest/tests/Harness_sanity/test_sanityEventUtils.html
@@ -29,30 +29,32 @@
 <p>blah blah blah blah</p>
 <p>blah blah blah blah</p>
 </div>
 <script class="testbody" type="text/javascript">
 const kStrictKeyPressEvents =
   SpecialPowers.getBoolPref("dom.keyboardevent.keypress.dispatch_non_printable_keys_only_system_group_in_content");
 const kStrictKeyDownKeyUpEvents =
   SpecialPowers.getBoolPref("dom.keyboardevent.dispatch_during_composition");
-const kIsHeadless =
-  SpecialPowers.Cc["@mozilla.org/gfx/info;1"].getService(SpecialPowers.Ci.nsIGfxInfo).isHeadless;
 
 info("\nProfile::EventUtilsLoadTime: " + (loadTime - start) + "\n");
 function starttest() {
   SimpleTest.waitForFocus(
     async function () {
       SimpleTest.waitForExplicitFinish();
       var startTime = new Date();
       var check = false;
       function doCheck() {
         check = true;
       }
 
+      const kIsHeadless = await SpecialPowers.spawnChrome([], () => {
+        return Cc["@mozilla.org/gfx/info;1"].getService(Ci.nsIGfxInfo).isHeadless;
+      });
+
       if (navigator.appVersion.includes("Android")) {
         // This is the workaround for test failure on debug build.
         await SpecialPowers.pushPrefEnv({set: [["apz.zoom-to-focused-input.enabled", false]]});
       }
 
       /* test send* functions */
       $("testMouseEvent").addEventListener("click", doCheck, {once: true});
       sendMouseEvent({type:'click'}, "testMouseEvent");