diff --git a/browser/base/content/browser.js b/browser/base/content/browser.js
index 59d10c875190..c504170e89d7 100755
--- a/browser/base/content/browser.js
+++ b/browser/base/content/browser.js
@@ -8717,7 +8717,7 @@ var ToolbarIconColor = {
   // to avoid unnecessary calls to getComputedStyle
   _toolbarLuminanceCache: new Map(),
 
-  inferFromText(reason, reasonValue) {
+  async inferFromText(reason, reasonValue) {
     if (!this._initialized)
       return;
     function parseRGB(aColorString) {
@@ -8754,20 +8754,22 @@ var ToolbarIconColor = {
     // 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)
diff --git a/browser/base/content/test/performance/browser_windowopen_reflows.js b/browser/base/content/test/performance/browser_windowopen_reflows.js
index f00ef85dc659..e7e85ee983e8 100644
--- a/browser/base/content/test/performance/browser_windowopen_reflows.js
+++ b/browser/base/content/test/performance/browser_windowopen_reflows.js
@@ -32,8 +32,10 @@ if (Services.appinfo.OS == "Linux") {
     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",
       ],
     );
   }
@@ -43,8 +45,10 @@ 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",
     ],
   );
 }
@@ -67,8 +71,10 @@ if (Services.appinfo.OS == "WINNT") {
 
     [
       "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",
     ],
 
     [