--- a/browser/base/content/browser.js
+++ b/browser/base/content/browser.js
@@ -8712,17 +8712,17 @@ var ToolbarIconColor = {
         break;
     }
   },
 
   // a cache of luminance values for each toolbar
   // to avoid unnecessary calls to getComputedStyle
   _toolbarLuminanceCache: new Map(),
 
-  inferFromText(reason, reasonValue) {
+  async inferFromText(reason, reasonValue) {
     if (!this._initialized)
       return;
     function parseRGB(aColorString) {
       let rgb = aColorString.match(/^rgba?\((\d+), (\d+), (\d+)/);
       rgb.shift();
       return rgb.map(x => parseInt(x));
     }
 
@@ -8749,30 +8749,32 @@ var ToolbarIconColor = {
     let toolbarSelector = "#navigator-toolbox > toolbar:not([collapsed=true]):not(#addon-bar)";
     if (AppConstants.platform == "macosx")
       toolbarSelector += ":not([type=menubar])";
 
     // The getComputedStyle calls and setting the brighttext are separated in
     // two loops to avoid flushing layout and making it dirty repeatedly.
     let cachedLuminances = this._toolbarLuminanceCache;
     let luminances = new Map();
-    for (let toolbar of document.querySelectorAll(toolbarSelector)) {
-      // toolbars *should* all have ids, but guard anyway to avoid blowing up
-      let cacheKey = toolbar.id && toolbar.id + JSON.stringify(this._windowState);
-      // lookup cached luminance value for this toolbar in this window state
-      let luminance = cacheKey && cachedLuminances.get(cacheKey);
-      if (isNaN(luminance)) {
-        let [r, g, b] = parseRGB(getComputedStyle(toolbar).color);
-        luminance = 0.2125 * r + 0.7154 * g + 0.0721 * b;
-        if (cacheKey) {
-          cachedLuminances.set(cacheKey, luminance);
+    await BrowserUtils.promiseLayoutFlushed(document, "style", () => {
+      for (let toolbar of document.querySelectorAll(toolbarSelector)) {
+        // toolbars *should* all have ids, but guard anyway to avoid blowing up
+        let cacheKey = toolbar.id && toolbar.id + JSON.stringify(this._windowState);
+        // lookup cached luminance value for this toolbar in this window state
+        let luminance = cacheKey && cachedLuminances.get(cacheKey);
+        if (isNaN(luminance)) {
+          let [r, g, b] = parseRGB(getComputedStyle(toolbar).color);
+          luminance = 0.2125 * r + 0.7154 * g + 0.0721 * b;
+          if (cacheKey) {
+            cachedLuminances.set(cacheKey, luminance);
+          }
         }
+        luminances.set(toolbar, luminance);
       }
-      luminances.set(toolbar, luminance);
-    }
+    });
 
     for (let [toolbar, luminance] of luminances) {
       if (luminance <= 110)
         toolbar.removeAttribute("brighttext");
       else
         toolbar.setAttribute("brighttext", "true");
     }
   }

--- a/browser/base/content/test/performance/browser_windowopen_reflows.js
+++ b/browser/base/content/test/performance/browser_windowopen_reflows.js
@@ -27,29 +27,33 @@ if (Services.appinfo.OS == "Linux") {
         "handleEvent@chrome://browser/content/tabbrowser.xml",
         "EventListener.handleEvent*tabbrowser-tabs_XBL_Constructor@chrome://browser/content/tabbrowser.xml",
       ],
     );
   } else {
     EXPECTED_REFLOWS.push(
       [
         "handleEvent@chrome://browser/content/tabbrowser.xml",
-        "inferFromText@chrome://browser/content/browser.js",
-        "handleEvent@chrome://browser/content/browser.js",
+        "inferFromText/<@chrome://browser/content/browser.js",
+        "promiseReflowed/</<@resource://gre/modules/BrowserUtils.jsm",
+        "_onReflow@resource://gre/modules/BrowserUtils.jsm",
+        "reflowInterruptible@resource://gre/modules/BrowserUtils.jsm",
       ],
     );
   }
 }
 
 if (Services.appinfo.OS == "Darwin") {
   EXPECTED_REFLOWS.push(
     [
       "handleEvent@chrome://browser/content/tabbrowser.xml",
-      "inferFromText@chrome://browser/content/browser.js",
-      "handleEvent@chrome://browser/content/browser.js",
+      "inferFromText/<@chrome://browser/content/browser.js",
+      "promiseReflowed/</<@resource://gre/modules/BrowserUtils.jsm",
+      "_onReflow@resource://gre/modules/BrowserUtils.jsm",
+      "reflowInterruptible@resource://gre/modules/BrowserUtils.jsm",
     ],
   );
 }
 
 if (Services.appinfo.OS == "WINNT") {
   EXPECTED_REFLOWS.push(
     [
       "verticalMargins@chrome://browser/content/browser-tabsintitlebar.js",
@@ -62,18 +66,20 @@ if (Services.appinfo.OS == "WINNT") {
       "verticalMargins@chrome://browser/content/browser-tabsintitlebar.js",
       "_update@chrome://browser/content/browser-tabsintitlebar.js",
       "updateAppearance@chrome://browser/content/browser-tabsintitlebar.js",
       "handleEvent@chrome://browser/content/tabbrowser.xml",
     ],
 
     [
       "handleEvent@chrome://browser/content/tabbrowser.xml",
-      "inferFromText@chrome://browser/content/browser.js",
-      "handleEvent@chrome://browser/content/browser.js",
+      "inferFromText/<@chrome://browser/content/browser.js",
+      "promiseReflowed/</<@resource://gre/modules/BrowserUtils.jsm",
+      "_onReflow@resource://gre/modules/BrowserUtils.jsm",
+      "reflowInterruptible@resource://gre/modules/BrowserUtils.jsm",
     ],
 
     [
       "handleEvent@chrome://browser/content/tabbrowser.xml",
       "EventListener.handleEvent*tabbrowser-tabs_XBL_Constructor@chrome://browser/content/tabbrowser.xml",
     ],
   );
 }