diff --git a/browser/base/content/test/about/browser_aboutCertError.js b/browser/base/content/test/about/browser_aboutCertError.js
index 47504f1a3da9..515a5d825d80 100644
--- a/browser/base/content/test/about/browser_aboutCertError.js
+++ b/browser/base/content/test/about/browser_aboutCertError.js
@@ -497,18 +497,26 @@ add_task(async function checkSandboxedIframe() {
   let bc = browser.browsingContext.children[0];
   await SpecialPowers.spawn(bc, [], async function() {
     let doc = content.document;
+
+    // aboutNetError.js is using async localization to format several messages
+    // and in result the translation may be applied later.
+    // We want to return the textContent of the element only after
+    // the translation completes, so let's wait for it here.
+    let elements = [
+      doc.querySelector(".title-text"),
+      doc.getElementById("errorCode"),
+    ];
+    await ContentTaskUtils.waitForCondition(() => {
+      return elements.every(elem => !!elem.textContent.trim().length);
+    });
+
     let titleText = doc.querySelector(".title-text");
     Assert.ok(
       titleText.textContent.endsWith("Security Issue"),
       "Title shows Did Not Connect: Potential Security Issue"
     );
 
-    // Wait until fluent sets the errorCode inner text.
-    let el;
-    await ContentTaskUtils.waitForCondition(() => {
-      el = doc.getElementById("errorCode");
-      return el.textContent != "";
-    }, "error code has been set inside the advanced button panel");
+    let el = doc.getElementById("errorCode");
 
     Assert.equal(
       el.textContent,
diff --git a/browser/base/content/test/about/browser_aboutCertError_exception.js b/browser/base/content/test/about/browser_aboutCertError_exception.js
index b8f33433e6d0..cb6a3de3eb61 100644
--- a/browser/base/content/test/about/browser_aboutCertError_exception.js
+++ b/browser/base/content/test/about/browser_aboutCertError_exception.js
@@ -138,6 +138,16 @@ add_task(async function checkBadStsCert() {
           : content.document;
         let advancedButton = doc.getElementById("advancedButton");
         advancedButton.click();
+
+        // aboutNetError.js is using async localization to format several messages
+        // and in result the translation may be applied later.
+        // We want to return the textContent of the element only after
+        // the translation completes, so let's wait for it here.
+        let elements = [doc.getElementById("badCertTechnicalInfo")];
+        await ContentTaskUtils.waitForCondition(() => {
+          return elements.every(elem => !!elem.textContent.trim().length);
+        });
+
         return doc.getElementById("badCertTechnicalInfo").textContent;
       }
     );
diff --git a/browser/base/content/test/about/browser_aboutNetError_csp_iframe.js b/browser/base/content/test/about/browser_aboutNetError_csp_iframe.js
index ee6a0d641c45..91f9e6260f32 100644
--- a/browser/base/content/test/about/browser_aboutNetError_csp_iframe.js
+++ b/browser/base/content/test/about/browser_aboutNetError_csp_iframe.js
@@ -92,6 +92,19 @@ async function setupPage(htmlPageName, blockedPage) {
   // to open the blocked page in a new window/tab
   await SpecialPowers.spawn(iframe, [], async function() {
     let doc = content.document;
+
+    // aboutNetError.js is using async localization to format several messages
+    // and in result the translation may be applied later.
+    // We want to return the textContent of the element only after
+    // the translation completes, so let's wait for it here.
+    let elements = [
+      doc.getElementById("errorLongDesc"),
+      doc.getElementById("openInNewWindowButton"),
+    ];
+    await ContentTaskUtils.waitForCondition(() => {
+      return elements.every(elem => !!elem.textContent.trim().length);
+    });
+
     let textLongDescription = doc.getElementById("errorLongDesc").textContent;
     Assert.ok(
       textLongDescription.includes(