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diff --git a/browser/base/content/test/tabs/browser_long_data_url_label_truncation.js b/browser/base/content/test/tabs/browser_long_data_url_label_truncation.js
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+/* Any copyright is dedicated to the Public Domain.
+   http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * Ensure that tab labels for base64 data: URLs are always truncated
+ * to ensure that we don't hang trying to paint really long overflown
+ * text runs.
+ * This becomes a performance issue with 1mb or so long data URIs;
+ * this test uses a much shorter one for simplicity's sake.
+ */
+add_task(async function test_ensure_truncation() {
+  const MOBY = `
+  <!DOCTYPE html>
+  <meta charset="utf-8">
+    Call me Ishmael. Some years ago—never mind how
+    long precisely—having little or no money in my purse, and nothing particular
+    to interest me on shore, I thought I would sail about a little and see the
+    watery part of the world. It is a way I have of driving off the spleen and
+    regulating the circulation. Whenever I find myself growing grim about the
+    mouth; whenever it is a damp, drizzly November in my soul; whenever I find
+    myself involuntarily pausing before coffin warehouses, and bringing up the
+    rear of every funeral I meet; and especially whenever my hypos get such an
+    upper hand of me, that it requires a strong moral principle to prevent me
+    from deliberately stepping into the street, and methodically knocking
+    people's hats off—then, I account it high time to get to sea as soon as I
+    can. This is my substitute for pistol and ball. With a philosophical
+    flourish Cato throws himself upon his sword; I quietly take to the ship.
+    There is nothing surprising in this. If they but knew it, almost all men in
+    their degree, some time or other, cherish very nearly the same feelings
+    towards the ocean with me.`;
+
+  let fileReader = new FileReader();
+  const DATA_URL = await new Promise(resolve => {
+    fileReader.addEventListener("load", e => resolve(fileReader.result));
+    fileReader.readAsDataURL(new Blob([MOBY], { type: "text/html" }));
+  });
+  // Substring the full URL to avoid log clutter because Assert will print
+  // the entire thing.
+  Assert.stringContains(
+    DATA_URL.substring(0, 30),
+    "base64",
+    "data URL needs to be base64"
+  );
+
+  let newTab;
+  function tabLabelChecker() {
+    Assert.lessOrEqual(
+      newTab.label.length,
+      501,
+      "Tab label should not exceed 500 chars + ellipsis."
+    );
+  }
+  let mutationObserver = new MutationObserver(tabLabelChecker);
+  registerCleanupFunction(() => mutationObserver.disconnect());
+
+  gBrowser.tabContainer.addEventListener(
+    "TabOpen",
+    event => {
+      newTab = event.target;
+      tabLabelChecker();
+      mutationObserver.observe(newTab, {
+        attributeFilter: ["label"],
+      });
+    },
+    { once: true }
+  );
+
+  await BrowserTestUtils.withNewTab(DATA_URL, async () => {
+    // Wait another longer-than-tick to ensure more mutation observer things have
+    // come in.
+    await new Promise(executeSoon);
+
+    // Check one last time for good measure, for the final label:
+    tabLabelChecker();
+  });
+});