--- a/browser/app/profile/firefox.js
+++ b/browser/app/profile/firefox.js
@@ -258,16 +258,22 @@ pref("browser.compactmode.show", false);
 
 // At startup, check if we're the default browser and prompt user if not.
 pref("browser.shell.checkDefaultBrowser", true);
 pref("browser.shell.shortcutFavicons",true);
 pref("browser.shell.mostRecentDateSetAsDefault", "");
 pref("browser.shell.skipDefaultBrowserCheckOnFirstRun", true);
 pref("browser.shell.didSkipDefaultBrowserCheckOnFirstRun", false);
 pref("browser.shell.defaultBrowserCheckCount", 0);
+#if defined(XP_WIN)
+// Attempt to set the default browser on Windows 10 using the UserChoice registry keys,
+// before falling back to launching the modern Settings dialog.
+pref("browser.shell.setDefaultBrowserUserChoice", true);
+#endif
+
 
 // 0 = blank, 1 = home (browser.startup.homepage), 2 = last visited page, 3 = resume previous browser session
 // The behavior of option 3 is detailed at: http://wiki.mozilla.org/Session_Restore
 pref("browser.startup.page",                1);
 pref("browser.startup.homepage",            "about:home");
 #ifdef NIGHTLY_BUILD
 pref("browser.startup.homepage.abouthome_cache.enabled", true);
 #else

--- a/browser/components/shell/ShellService.jsm
+++ b/browser/components/shell/ShellService.jsm
@@ -14,16 +14,28 @@ const { XPCOMUtils } = ChromeUtils.impor
   "resource://gre/modules/XPCOMUtils.jsm"
 );
 ChromeUtils.defineModuleGetter(
   this,
   "WindowsRegistry",
   "resource://gre/modules/WindowsRegistry.jsm"
 );
 
+XPCOMUtils.defineLazyModuleGetters(this, {
+  Subprocess: "resource://gre/modules/Subprocess.jsm",
+  setTimeout: "resource://gre/modules/Timer.jsm",
+});
+
+XPCOMUtils.defineLazyServiceGetter(
+  this,
+  "XreDirProvider",
+  "@mozilla.org/xre/directory-provider;1",
+  "nsIXREDirProvider"
+);
+
 /**
  * Internal functionality to save and restore the docShell.allow* properties.
  */
 let ShellServiceInternal = {
   /**
    * Used to determine whether or not to offer "Set as desktop background"
    * functionality. Even if shell service is available it is not
    * guaranteed that it is able to set the background for every desktop
@@ -107,16 +119,94 @@ let ShellServiceInternal = {
       this._checkedThisSession = true;
     }
     if (this.shellService) {
       return this.shellService.isDefaultBrowser(forAllTypes);
     }
     return false;
   },
 
+  /*
+   * Set the default browser through the UserChoice registry keys on Windows.
+   *
+   * NOTE: This does NOT open the System Settings app for manual selection
+   * in case of failure. If that is desired, catch the exception and call
+   * setDefaultBrowser().
+   *
+   * @return Promise, resolves when successful, rejects with Error on failure.
+   */
+  async setAsDefaultUserChoice() {
+    if (AppConstants.platform != "win") {
+      throw new Error("Windows-only");
+    }
+
+    // We launch the WDBA to handle the registry writes, see
+    // SetDefaultBrowserUserChoice() in
+    // toolkit/mozapps/defaultagent/SetDefaultBrowser.cpp.
+    // This is external in case an overzealous antimalware product decides to
+    // quarrantine any program that writes UserChoice, though this has not
+    // occurred during extensive testing.
+
+    if (!ShellService.checkAllProgIDsExist()) {
+      throw new Error("checkAllProgIDsExist() failed");
+    }
+
+    if (!ShellService.checkBrowserUserChoiceHashes()) {
+      throw new Error("checkBrowserUserChoiceHashes() failed");
+    }
+
+    const wdba = Services.dirsvc.get("XREExeF", Ci.nsIFile);
+    wdba.leafName = "default-browser-agent.exe";
+    const aumi = XreDirProvider.getInstallHash();
+
+    const exeProcess = await Subprocess.call({
+      command: wdba.path,
+      arguments: ["set-default-browser-user-choice", aumi],
+    });
+
+    // exit codes
+    const S_OK = 0;
+    const STILL_ACTIVE = 0x103;
+
+    const exeWaitTimeoutMs = 2000; // 2 seconds
+    const exeWaitPromise = exeProcess.wait();
+    const timeoutPromise = new Promise(function(resolve, reject) {
+      setTimeout(() => resolve({ exitCode: STILL_ACTIVE }), exeWaitTimeoutMs);
+    });
+    const { exitCode } = await Promise.race([exeWaitPromise, timeoutPromise]);
+
+    if (exitCode != S_OK) {
+      throw new Error(`WDBA nonzero exit code ${exitCode}`);
+    }
+  },
+
+  // override nsIShellService.setDefaultBrowser() on the ShellService proxy.
+  setDefaultBrowser(claimAllTypes, forAllUsers) {
+    // On Windows 10, our best chance is to set UserChoice, so try that first.
+    if (
+      AppConstants.isPlatformAndVersionAtLeast("win", "10") &&
+      Services.prefs.getBoolPref(
+        "browser.shell.setDefaultBrowserUserChoice",
+        true
+      )
+    ) {
+      // nsWindowsShellService::SetDefaultBrowser() kicks off several
+      // operations, but doesn't wait for their result. So we don't need to
+      // await the result of setAsDefaultUserChoice() here, either, we just need
+      // to fall back in case it fails.
+      this.setAsDefaultUserChoice().catch(err => {
+        Cu.reportError(err);
+        this.shellService.setDefaultBrowser(claimAllTypes, forAllUsers);
+      });
+      return;
+    }
+
+    this.shellService.setDefaultBrowser(claimAllTypes, forAllUsers);
+  },
+
   setAsDefault() {
     let claimAllTypes = true;
     let setAsDefaultError = false;
     if (AppConstants.platform == "win") {
       try {
         // In Windows 8+, the UI for selecting default protocol is much
         // nicer than the UI for setting file type associations. So we
         // only show the protocol association screen on Windows 8+.

--- a/browser/components/shell/moz.build
+++ b/browser/components/shell/moz.build
@@ -46,36 +46,44 @@ elif CONFIG["MOZ_WIDGET_TOOLKIT"] == "gt
 
 elif CONFIG["OS_ARCH"] == "WINNT":
     XPIDL_SOURCES += [
         "nsIWindowsShellService.idl",
     ]
     SOURCES += [
         "nsWindowsShellService.cpp",
         "WindowsDefaultBrowser.cpp",
+        "WindowsUserChoice.cpp",
     ]
     LOCAL_INCLUDES += [
         "../../../other-licenses/nsis/Contrib/CityHash/cityhash",
     ]
     OS_LIBS += [
+        "bcrypt",
+        "crypt32",
         "propsys",
     ]
 
 XPIDL_MODULE = "shellservice"
 
 if SOURCES:
     FINAL_LIBRARY = "browsercomps"
 
 EXTRA_JS_MODULES += [
     "HeadlessShell.jsm",
     "ScreenshotChild.jsm",
     "ShellService.jsm",
 ]
 
-for var in ("MOZ_APP_DISPLAYNAME", "MOZ_APP_NAME", "MOZ_APP_VERSION"):
+for var in (
+    "MOZ_APP_DISPLAYNAME",
+    "MOZ_APP_NAME",
+    "MOZ_APP_VERSION",
+    "MOZ_DEFAULT_BROWSER_AGENT",
+):
     DEFINES[var] = '"%s"' % CONFIG[var]
 
 CXXFLAGS += CONFIG["TK_CFLAGS"]
 if CONFIG["MOZ_ENABLE_DBUS"]:
     CXXFLAGS += CONFIG["MOZ_DBUS_GLIB_CFLAGS"]
 
 with Files("**"):
     BUG_COMPONENT = ("Firefox", "Shell Integration")

--- a/browser/components/shell/nsIWindowsShellService.idl
+++ b/browser/components/shell/nsIWindowsShellService.idl
@@ -84,9 +84,28 @@ interface nsIWindowsShellService : nsISu
    * - "Taskbar", Taskbar Pins
    * - "" otherwise
    *
    * NOTE: This tries to avoid I/O, so paths are compared directly as
    * strings, which may not be accurate in all cases. It is intended
    * for noncritical telemetry use.
    */
   AString classifyShortcut(in AString aPath);
+
+  /*
+   * Check if setDefaultBrowserUserChoice() is expected to succeed.
+   *
+   * This checks the ProgIDs for this installation, and the hash of the existing
+   * UserChoice association.
+   *
+   * @return true if the check succeeds, false otherwise.
+   */
+  bool canSetDefaultBrowserUserChoice();
+
+  /*
+   * checkAllProgIDsExist() and checkBrowserUserChoiceHashes() are components
+   * of canSetDefaultBrowserUserChoice(), broken out for telemetry purposes.
+   *
+   * @return true if the check succeeds, false otherwise.
+   */
+  bool checkAllProgIDsExist();
+  bool checkBrowserUserChoiceHashes();
 };

--- a/browser/components/shell/nsWindowsShellService.cpp
+++ b/browser/components/shell/nsWindowsShellService.cpp
@@ -26,16 +26,17 @@
 #include "nsIURLFormatter.h"
 #include "nsXULAppAPI.h"
 #include "mozilla/WindowsVersion.h"
 #include "mozilla/dom/Element.h"
 #include "mozilla/dom/Promise.h"
 #include "mozilla/ErrorResult.h"
 #include "mozilla/gfx/2D.h"
 #include "WindowsDefaultBrowser.h"
+#include "WindowsUserChoice.h"
 
 #include <windows.h>
 #include <shellapi.h>
 #include <propvarutil.h>
 #include <propkey.h>
 
 #ifdef __MINGW32__
 // MinGW-w64 headers are missing PropVariantToString.
@@ -226,16 +227,49 @@ nsresult nsWindowsShellService::LaunchCo
   }
   return SUCCEEDED(hr) ? NS_OK : NS_ERROR_FAILURE;
 }
 
 nsresult nsWindowsShellService::LaunchControlPanelDefaultPrograms() {
   return ::LaunchControlPanelDefaultPrograms() ? NS_OK : NS_ERROR_FAILURE;
 }
 
+NS_IMETHODIMP
+nsWindowsShellService::CheckAllProgIDsExist(bool* aResult) {
+  *aResult = false;
+  nsAutoString aumid;
+  if (!mozilla::widget::WinTaskbar::GetAppUserModelID(aumid)) {
+    return NS_OK;
+  }
+  *aResult =
+      CheckProgIDExists(FormatProgID(L"FirefoxURL", aumid.get()).get()) &&
+      CheckProgIDExists(FormatProgID(L"FirefoxHTML", aumid.get()).get());
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsWindowsShellService::CheckBrowserUserChoiceHashes(bool* aResult) {
+  *aResult = ::CheckBrowserUserChoiceHashes();
+  return NS_OK;
+}
+
+NS_IMETHODIMP
+nsWindowsShellService::CanSetDefaultBrowserUserChoice(bool* aResult) {
+  *aResult = false;
+// If the WDBA is not available, this could never succeed.
+#ifdef MOZ_DEFAULT_BROWSER_AGENT
+  bool progIDsExist = false;
+  bool hashOk = false;
+  *aResult = NS_SUCCEEDED(CheckAllProgIDsExist(&progIDsExist)) &&
+             progIDsExist &&
+             NS_SUCCEEDED(CheckBrowserUserChoiceHashes(&hashOk)) && hashOk;
+#endif
+  return NS_OK;
+}
+
 nsresult nsWindowsShellService::LaunchModernSettingsDialogDefaultApps() {
   return ::LaunchModernSettingsDialogDefaultApps() ? NS_OK : NS_ERROR_FAILURE;
 }
 
 nsresult nsWindowsShellService::InvokeHTTPOpenAsVerb() {
   nsCOMPtr<nsIURLFormatter> formatter(
       do_GetService("@mozilla.org/toolkit/URLFormatterService;1"));
   if (!formatter) {