diff --git a/browser/components/urlbar/UrlbarResult.jsm b/browser/components/urlbar/UrlbarResult.jsm
index 69db09329a25..311cf9f4ab20 100644
--- a/browser/components/urlbar/UrlbarResult.jsm
+++ b/browser/components/urlbar/UrlbarResult.jsm
@@ -177,6 +177,14 @@ class UrlbarResult {
       payloadInfo.displayUrl[0] = Services.textToSubURI.unEscapeURIForUI("UTF-8", url);
     }
 
+    // For performance reasons limit excessive string lengths, to reduce the
+    // amount of string matching we do here, and avoid wasting resources to
+    // handle long textruns that the user would never see anyway.
+    for (let prop of ["displayUrl", "title"].filter(p => p in payloadInfo)) {
+      payloadInfo[prop][0] =
+        payloadInfo[prop][0].substring(0, UrlbarUtils.MAX_TEXT_LENGTH);
+    }
+
     let entries = Object.entries(payloadInfo);
     return [
       entries.reduce((payload, [name, [val, _]]) => {
diff --git a/browser/components/urlbar/UrlbarUtils.jsm b/browser/components/urlbar/UrlbarUtils.jsm
index 7bd9da0e3afd..5b502989cc17 100644
--- a/browser/components/urlbar/UrlbarUtils.jsm
+++ b/browser/components/urlbar/UrlbarUtils.jsm
@@ -127,6 +127,10 @@ var UrlbarUtils = {
     BLUR: 2,
   },
 
+  // Limit the length of titles and URLs we display so layout doesn't spend too
+  // much time building text runs.
+  MAX_TEXT_LENGTH: 255,
+
   /**
    * Adds a url to history as long as it isn't in a private browsing window,
    * and it is valid.
diff --git a/browser/components/urlbar/tests/browser/browser.ini b/browser/components/urlbar/tests/browser/browser.ini
index 0ffe977f6d16..02f066775dfc 100644
--- a/browser/components/urlbar/tests/browser/browser.ini
+++ b/browser/components/urlbar/tests/browser/browser.ini
@@ -84,6 +84,7 @@ skip-if = os == 'linux' # Bug 1104755 (Intermittent failure)
 [browser_tabMatchesInAwesomebar.js]
 support-files =
   moz.png
+[browser_textruns.js]
 [browser_urlbar_blanking.js]
 support-files =
   file_blank_but_not_blank.html
diff --git a/browser/components/urlbar/tests/browser/browser_UrlbarInput_tooltip.js b/browser/components/urlbar/tests/browser/browser_UrlbarInput_tooltip.js
index 298c2639b8ab..ff3108e85be2 100644
--- a/browser/components/urlbar/tests/browser/browser_UrlbarInput_tooltip.js
+++ b/browser/components/urlbar/tests/browser/browser_UrlbarInput_tooltip.js
@@ -4,24 +4,20 @@
 "use strict";
 
 function synthesizeMouseOver(element) {
+  info("synthesize mouseover");
   let promise = BrowserTestUtils.waitForEvent(element, "mouseover");
-
-  EventUtils.synthesizeMouse(element, 1, 1, {type: "mouseover"});
-  EventUtils.synthesizeMouse(element, 2, 2, {type: "mousemove"});
-  EventUtils.synthesizeMouse(element, 3, 3, {type: "mousemove"});
-  EventUtils.synthesizeMouse(element, 4, 4, {type: "mousemove"});
-
+  EventUtils.synthesizeMouseAtCenter(document.documentElement, {type: "mouseout"});
+  EventUtils.synthesizeMouseAtCenter(element, {type: "mouseover"});
+  EventUtils.synthesizeMouseAtCenter(element, {type: "mousemove"});
   return promise;
 }
 
 function synthesizeMouseOut(element) {
+  info("synthesize mouseout");
   let promise = BrowserTestUtils.waitForEvent(element, "mouseout");
-
-  EventUtils.synthesizeMouse(element, 0, 0, {type: "mouseout"});
+  EventUtils.synthesizeMouseAtCenter(element, {type: "mouseover"});
+  EventUtils.synthesizeMouseAtCenter(element, {type: "mouseout"});
   EventUtils.synthesizeMouseAtCenter(document.documentElement, {type: "mousemove"});
-  EventUtils.synthesizeMouseAtCenter(document.documentElement, {type: "mousemove"});
-  EventUtils.synthesizeMouseAtCenter(document.documentElement, {type: "mousemove"});
-
   return promise;
 }
 
@@ -36,6 +32,7 @@ async function expectTooltip(text) {
 
   let popupShownPromise = BrowserTestUtils.waitForEvent(tooltip, "popupshown");
   await synthesizeMouseOver(element);
+  info("awaiting for tooltip popup");
   await popupShownPromise;
 
   is(element.getAttribute("title"), text, "title attribute has expected text");
@@ -66,7 +63,6 @@ add_task(async function() {
 
   // Ensure the URL bar is neither focused nor hovered before we start.
   gBrowser.selectedBrowser.focus();
-  await synthesizeMouseOver(gURLBar.inputField);
   await synthesizeMouseOut(gURLBar.inputField);
 
   gURLBar.value = "short string";
diff --git a/browser/components/urlbar/tests/browser/browser_textruns.js b/browser/components/urlbar/tests/browser/browser_textruns.js
new file mode 100644
index 000000000000..c83548905bfd
--- /dev/null
+++ b/browser/components/urlbar/tests/browser/browser_textruns.js
@@ -0,0 +1,26 @@
+/* Any copyright is dedicated to the Public Domain.
+ * http://creativecommons.org/publicdomain/zero/1.0/ */
+
+"use strict";
+
+/**
+ * This test ensures that we limit textruns in case of very long urls or titles.
+ */
+
+ add_task(async function() {
+  let lotsOfSpaces = "%20".repeat(300);
+  await PlacesTestUtils.addVisits({
+    uri: `https://textruns.mozilla.org/${lotsOfSpaces}/test/`,
+    title: `A long ${lotsOfSpaces} title`});
+  registerCleanupFunction(async function() {
+    await PlacesUtils.history.clear();
+  });
+
+  await promiseAutocompleteResultPopup("textruns");
+  let result = await UrlbarTestUtils.getDetailsOfResultAt(window, 1);
+  Assert.equal(result.displayed.title.length, UrlbarUtils.MAX_TEXT_LENGTH,
+               "Result title should be limited");
+  Assert.equal(result.displayed.url.length, UrlbarUtils.MAX_TEXT_LENGTH,
+               "Result url should be limited");
+  await UrlbarTestUtils.promisePopupClose(window);
+});
diff --git a/browser/components/urlbar/tests/legacy/browser.ini b/browser/components/urlbar/tests/legacy/browser.ini
index 1079fdd91c2a..d320f228560b 100644
--- a/browser/components/urlbar/tests/legacy/browser.ini
+++ b/browser/components/urlbar/tests/legacy/browser.ini
@@ -100,6 +100,7 @@ skip-if = os == 'linux' # Bug 1104755
 [../browser/browser_tabMatchesInAwesomebar.js]
 support-files =
   ../browser/moz.png
+[../browser/browser_textruns.js]
 [../browser/browser_urlbarAboutHomeLoading.js]
 [../browser/browser_urlbarCopying.js]
 subsuite = clipboard