--- a/browser/base/content/test/performance/browser.ini
+++ b/browser/base/content/test/performance/browser.ini
@@ -9,35 +9,29 @@ prefs =
   # representative of common startup.
   browser.migration.version=9999999
   browser.urlbar.quantumbar=true
   browser.startup.record=true
   gfx.canvas.willReadFrequently.enable=true
   # The form autofill framescript is only used in certain locales if this
   # pref is set to 'detect', which is the default value on non-Nightly.
   extensions.formautofill.available='on'
-environment =
-  MOZ_PROFILER_STARTUP=1
-  MOZ_PROFILER_STARTUP_FEATURES=js,mainthreadio
-  MOZ_PROFILER_STARTUP_ENTRIES=10000000
 support-files =
   head.js
 [browser_appmenu.js]
 skip-if = asan || debug || (os == 'win' && bits == 32) || (os == 'win' && processor == 'aarch64') # Bug 1382809, bug 1369959, Win32 because of intermittent OOM failures, bug 1533141 for aarch64
 [browser_preferences_usage.js]
 skip-if = !debug
 [browser_startup.js]
 [browser_startup_content.js]
 skip-if = !e10s
 [browser_startup_flicker.js]
 run-if = debug || devedition || nightly_build # Requires startupRecorder.js, which isn't shipped everywhere by default
 [browser_startup_hiddenwindow.js]
 skip-if = os == 'mac'
-[browser_startup_mainthreadio.js]
-[browser_startup_content_mainthreadio.js]
 [browser_tabclose_grow.js]
 [browser_tabclose.js]
 skip-if = (os == 'win') || (os == 'mac') # Bug 1488537, Bug 1531417, Bug 1497713
 [browser_tabdetach.js]
 [browser_tabopen.js]
 skip-if = (verify && (os == 'mac'))
 [browser_tabopen_squeeze.js]
 [browser_tabstrip_overflow_underflow.js]

new file mode 100644
--- /dev/null
+++ b/browser/base/content/test/performance/io/browser.ini
@@ -0,0 +1,20 @@
+[DEFAULT]
+# Currently disabled on debug due to debug-only failures, see bug 1549723.
+skip-if = debug
+# to avoid overhead when running the browser normally, startupRecorder.js will
+# do almost nothing unless browser.startup.record is true.
+# gfx.canvas.willReadFrequently.enable is just an optimization, but needs to be
+# set during early startup to have an impact as a canvas will be used by
+# startupRecorder.js
+prefs =
+  # Skip migration work in BG__migrateUI for browser_startup.js since it isn't
+  # representative of common startup, and triggers Places I/O.
+  browser.migration.version=9999999
+  browser.startup.record=true
+  gfx.canvas.willReadFrequently.enable=true
+environment =
+  MOZ_PROFILER_STARTUP=1
+  MOZ_PROFILER_STARTUP_FEATURES=js,mainthreadio
+  MOZ_PROFILER_STARTUP_ENTRIES=10000000
+[../browser_startup_mainthreadio.js]
+[../browser_startup_content_mainthreadio.js]

--- a/browser/base/moz.build
+++ b/browser/base/moz.build
@@ -29,16 +29,17 @@ BROWSER_CHROME_MANIFESTS += [
     'content/test/historySwipeAnimation/browser.ini',
     'content/test/keyboard/browser.ini',
     'content/test/menubar/browser.ini',
     'content/test/metaTags/browser.ini',
     'content/test/pageActions/browser.ini',
     'content/test/pageinfo/browser.ini',
     'content/test/performance/browser.ini',
     'content/test/performance/hidpi/browser.ini',
+    'content/test/performance/io/browser.ini',
     'content/test/performance/legacyurlbar/browser.ini',
     'content/test/performance/lowdpi/browser.ini',
     'content/test/permissions/browser.ini',
     'content/test/plugins/browser.ini',
     'content/test/popupNotifications/browser.ini',
     'content/test/popups/browser.ini',
     'content/test/referrer/browser.ini',
     'content/test/sanitize/browser.ini',

--- a/browser/components/tests/startupRecorder.js
+++ b/browser/components/tests/startupRecorder.js
@@ -160,16 +160,22 @@ startupRecorder.prototype = {
         win = null;
         this.data.frames = paints;
         this.data.prefStats = {};
         if (AppConstants.DEBUG) {
           Services.prefs.readStats((key, value) => this.data.prefStats[key] = value);
         }
         paints = null;
 
+        let env = Cc["@mozilla.org/process/environment;1"].getService(Ci.nsIEnvironment);
+        if (!env.exists("MOZ_PROFILER_STARTUP")) {
+          this._resolve();
+          this._resolve = null;
+          return;
+        }
 
         Services.profiler.getProfileDataAsync().then(profileData => {
           this.data.profile = profileData;
           // There's no equivalent StartProfiler call in this file because the
           // profiler is started using the MOZ_PROFILER_STARTUP environment
           // variable in browser/base/content/test/performance/browser.ini
           Services.profiler.StopProfiler();