--- a/toolkit/components/extensions/test/mochitest/test_ext_cookies_first_party.html
+++ b/toolkit/components/extensions/test/mochitest/test_ext_cookies_first_party.html
@@ -5,16 +5,18 @@
 <script src="/tests/SimpleTest/ExtensionTestUtils.js"></script>
 <script src="head.js"></script>
 <script>
 "use strict";
 
 async function background() {
   const url = "http://ext-cookie-first-party.mochi.test/";
   const firstPartyDomain = "ext-cookie-first-party.mochi.test";
+  // A first party domain with invalid characters for the file system, which just happens to be a IPv6 address.
+  const firstPartyDomainInvalidChars = "[2606:4700:4700::1111]";
   const expectedError = "First-Party Isolation is enabled, but the required 'firstPartyDomain' attribute was not set.";
 
   const assertExpectedCookies = (expected, cookies, message) => {
     let matches = (cookie, expected) => {
       if (!cookie || !expected) {
         return cookie === expected; // true if both are null.
       }
       for (let key of Object.keys(expected)) {
@@ -211,16 +213,48 @@ async function background() {
     ], [cookie], "remove: FPI on, w/ firstPartyDomain, FP cookie (set when FPI off)");
 
     // Test if FP cookies set when FPI on can be accessed when FPI off.
     await browser.cookies.set({url, name: "foo4", value: "bar4", firstPartyDomain});
 
     browser.test.sendMessage("test_fpi_enabled");
   };
 
+  // Test FPI with a first party domain with invalid characters for
+  // the file system.
+  const test_fpi_with_invalid_characters = async () => {
+    let cookie;
+
+    // Test setting a cookie with a first party domain with invalid characters
+    // for the file system.
+    cookie = await browser.cookies.set({url, name: "foo5", value: "bar5",
+                                        firstPartyDomain: firstPartyDomainInvalidChars});
+    assertExpectedCookies([
+      {name: "foo5", value: "bar5", firstPartyDomain: firstPartyDomainInvalidChars},
+    ], [cookie], "set: FPI on, w/ firstPartyDomain with invalid characters, FP cookie");
+
+    // Test getting a cookie with a first party domain with invalid characters
+    // for the file system.
+    cookie = await browser.cookies.get({url, name: "foo5",
+                                        firstPartyDomain: firstPartyDomainInvalidChars});
+    assertExpectedCookies([
+      {name: "foo5", value: "bar5", firstPartyDomain: firstPartyDomainInvalidChars},
+    ], [cookie], "get: FPI on, w/ firstPartyDomain with invalid characters, FP cookie");
+
+    // Test removing a cookie with a first party domain with invalid characters
+    // for the file system.
+    cookie = await browser.cookies.remove({url, name: "foo5",
+                                           firstPartyDomain: firstPartyDomainInvalidChars});
+    assertExpectedCookies([
+      {url, name: "foo5", firstPartyDomain: firstPartyDomainInvalidChars},
+    ], [cookie], "remove: FPI on, w/ firstPartyDomain with invalid characters, FP cookie");
+
+    browser.test.sendMessage("test_fpi_with_invalid_characters");
+  };
+
   // Test when FPI is disabled again, accessing FP cookies set when FPI is enabled.
   const test_fpd_cookies_on_fpi_disabled = async () => {
     let cookie, cookies;
     cookie = await browser.cookies.get({url, name: "foo4", firstPartyDomain});
     assertExpectedCookies([
       {name: "foo4", value: "bar4", firstPartyDomain},
     ], [cookie], "get: FPI off, w/ firstPartyDomain, FP cookie (set when FPI on)");
     cookie = await browser.cookies.remove({url, name: "foo4", firstPartyDomain});
@@ -236,16 +270,17 @@ async function background() {
 
     browser.test.sendMessage("test_fpd_cookies_on_fpi_disabled");
   };
 
   browser.test.onMessage.addListener((message) => {
     switch (message) {
       case "test_fpi_disabled": return test_fpi_disabled();
       case "test_fpi_enabled": return test_fpi_enabled();
+      case "test_fpi_with_invalid_characters": return test_fpi_with_invalid_characters();
       case "test_fpd_cookies_on_fpi_disabled": return test_fpd_cookies_on_fpi_disabled();
       default: return browser.test.notifyFail("unknown-message");
     }
   });
 }
 
 function enableFirstPartyIsolation() {
   return SpecialPowers.pushPrefEnv({
@@ -267,14 +302,16 @@ add_task(async () => {
     },
   });
   await extension.startup();
   extension.sendMessage("test_fpi_disabled");
   await extension.awaitMessage("test_fpi_disabled");
   await enableFirstPartyIsolation();
   extension.sendMessage("test_fpi_enabled");
   await extension.awaitMessage("test_fpi_enabled");
+  extension.sendMessage("test_fpi_with_invalid_characters");
+  await extension.awaitMessage("test_fpi_with_invalid_characters");
   await disableFirstPartyIsolation();
   extension.sendMessage("test_fpd_cookies_on_fpi_disabled");
   await extension.awaitMessage("test_fpd_cookies_on_fpi_disabled");
   await extension.unload();
 });
 </script>