diff --git a/browser/components/resistfingerprinting/test/browser/browser.ini b/browser/components/resistfingerprinting/test/browser/browser.ini
index 0b879410b34d..65e19604d1a5 100644
--- a/browser/components/resistfingerprinting/test/browser/browser.ini
+++ b/browser/components/resistfingerprinting/test/browser/browser.ini
@@ -2,6 +2,7 @@
 tags = resistfingerprinting
 support-files =
   browser_navigator_header.sjs
+  coop_header.sjs
   file_dummy.html
   file_keyBoardEvent.sjs
   file_navigator.html
@@ -34,4 +35,5 @@ skip-if = os == "linux" && bits == 64 && os_version == "18.04"
 [browser_spoofing_keyboard_event.js]
 skip-if = (debug || asan) && os == "linux" && bits == 64 #Bug 1518179
 [browser_timezone.js]
+[browser_timing_precision_cross_origin_isolated.js]
 [browser_bug1369357_site_specific_zoom_level.js]
diff --git a/browser/components/resistfingerprinting/test/browser/browser_performanceAPI.js b/browser/components/resistfingerprinting/test/browser/browser_performanceAPI.js
index 96afa0f745e2..e11d58d8b2a3 100644
--- a/browser/components/resistfingerprinting/test/browser/browser_performanceAPI.js
+++ b/browser/components/resistfingerprinting/test/browser/browser_performanceAPI.js
@@ -80,6 +80,7 @@ let isRounded = (x, expectedPrecision) => {
 let setupTest = async function(
   resistFingerprinting,
   reduceTimerPrecision,
+  crossOriginIsolated,
   expectedPrecision,
   runTests,
   workerCall
@@ -92,14 +93,19 @@ let setupTest = async function(
         "privacy.resistFingerprinting.reduceTimerPrecision.microseconds",
         expectedPrecision * 1000,
       ],
+      ["browser.tabs.remote.useCrossOriginOpenerPolicy", crossOriginIsolated],
+      ["browser.tabs.remote.useCrossOriginEmbedderPolicy", crossOriginIsolated],
+      ["browser.tabs.documentchannel", crossOriginIsolated],
     ],
   });
 
+  let url = crossOriginIsolated
+    ? `https://example.com/browser/browser/components/resistfingerprinting` +
+      `/test/browser/coop_header.sjs?crossOriginIsolated=${crossOriginIsolated}`
+    : TEST_PATH + "file_dummy.html";
+
   let win = await BrowserTestUtils.openNewBrowserWindow();
-  let tab = await BrowserTestUtils.openNewForegroundTab(
-    win.gBrowser,
-    TEST_PATH + "file_dummy.html"
-  );
+  let tab = await BrowserTestUtils.openNewForegroundTab(win.gBrowser, url);
 
   // No matter what we set the precision to, if we're in ResistFingerprinting mode
   // we use the larger of the precision pref and the constant 100ms
@@ -118,6 +124,15 @@ let setupTest = async function(
     ],
     runTests
   );
+
+  if (crossOriginIsolated) {
+    let remoteType = tab.linkedBrowser.remoteType;
+    ok(
+      remoteType.startsWith(E10SUtils.WEB_REMOTE_COOP_COEP_TYPE_PREFIX),
+      `${remoteType} expected to be coop+coep`
+    );
+  }
+
   await BrowserTestUtils.closeWindow(win);
 };
 // ================================================================================================
@@ -181,10 +196,13 @@ add_task(async function runRPTests() {
     );
   };
 
-  await setupTest(true, true, 200, runTests);
-  await setupTest(true, true, 100, runTests);
-  await setupTest(true, false, 13, runTests);
-  await setupTest(true, false, 0.13, runTests);
+  await setupTest(true, true, false, 200, runTests);
+  await setupTest(true, true, false, 100, runTests);
+  await setupTest(true, false, false, 13, runTests);
+  await setupTest(true, false, false, 0.13, runTests);
+  await setupTest(true, true, true, 100, runTests);
+  await setupTest(true, false, true, 13, runTests);
+  await setupTest(true, false, true, 0.13, runTests);
 });
 
 // ================================================================================================
@@ -264,9 +282,10 @@ add_task(async function runRTPTests() {
     content.performance.clearResourceTimings();
   };
 
-  await setupTest(false, true, 100, runTests);
-  await setupTest(false, true, 13, runTests);
-  await setupTest(false, true, 0.13, runTests);
+  await setupTest(false, true, false, 100, runTests);
+  await setupTest(false, true, false, 13, runTests);
+  await setupTest(false, true, false, 0.13, runTests);
+  await setupTest(false, true, true, 0.005, runTests);
 });
 
 // ================================================================================================
@@ -293,13 +312,13 @@ let runWorkerTest = async function(data) {
 };
 
 add_task(async function runRPTestsForWorker() {
-  await setupTest(true, true, 100, runWorkerTest, "runRPTests");
-  await setupTest(true, false, 13, runWorkerTest, "runRPTests");
-  await setupTest(true, true, 0.13, runWorkerTest, "runRPTests");
+  await setupTest(true, true, false, 100, runWorkerTest, "runRPTests");
+  await setupTest(true, false, false, 13, runWorkerTest, "runRPTests");
+  await setupTest(true, true, false, 0.13, runWorkerTest, "runRPTests");
 });
 
 add_task(async function runRTPTestsForWorker() {
-  await setupTest(false, true, 100, runWorkerTest, "runRTPTests");
-  await setupTest(false, true, 13, runWorkerTest, "runRTPTests");
-  await setupTest(false, true, 0.13, runWorkerTest, "runRTPTests");
+  await setupTest(false, true, false, 100, runWorkerTest, "runRTPTests");
+  await setupTest(false, true, false, 13, runWorkerTest, "runRTPTests");
+  await setupTest(false, true, false, 0.13, runWorkerTest, "runRTPTests");
 });
diff --git a/browser/components/resistfingerprinting/test/browser/browser_timing_precision_cross_origin_isolated.js b/browser/components/resistfingerprinting/test/browser/browser_timing_precision_cross_origin_isolated.js
new file mode 100644
index 000000000000..d93aa2af03a8
--- /dev/null
+++ b/browser/components/resistfingerprinting/test/browser/browser_timing_precision_cross_origin_isolated.js
@@ -0,0 +1,381 @@
+/**
+ * Bug 1621677 - A test for making sure getting the correct (higher) precision
+ *   when it's cross-origin-isolated.
+ */
+
+let isRounded = (x, expectedPrecision) => {
+  let rounded = Math.floor(x / expectedPrecision) * expectedPrecision;
+  // First we do the perfectly normal check that should work just fine
+  if (rounded === x || x === 0) {
+    return true;
+  }
+
+  // When we're diving by non-whole numbers, we may not get perfect
+  // multiplication/division because of floating points.
+  // When dealing with ms since epoch, a double's precision is on the order
+  // of 1/5 of a microsecond, so we use a value a little higher than that as
+  // our epsilon.
+  // To be clear, this error is introduced in our re-calculation of 'rounded'
+  // above in JavaScript.
+  if (Math.abs(rounded - x + expectedPrecision) < 0.0005) {
+    return true;
+  } else if (Math.abs(rounded - x) < 0.0005) {
+    return true;
+  }
+
+  // Then we handle the case where you're sub-millisecond and the timer is not
+  // We check that the timer is not sub-millisecond by assuming it is not if it
+  // returns an even number of milliseconds
+  if (expectedPrecision < 1 && Math.round(x) == x) {
+    if (Math.round(rounded) == x) {
+      return true;
+    }
+  }
+
+  ok(
+    false,
+    "Looming Test Failure, Additional Debugging Info: Expected Precision: " +
+      expectedPrecision +
+      " Measured Value: " +
+      x +
+      " Rounded Vaue: " +
+      rounded +
+      " Fuzzy1: " +
+      Math.abs(rounded - x + expectedPrecision) +
+      " Fuzzy 2: " +
+      Math.abs(rounded - x)
+  );
+
+  return false;
+};
+
+let setupTest = async function(
+  resistFingerprinting,
+  reduceTimerPrecision,
+  crossOriginIsolated,
+  expectedPrecision,
+  runTests,
+  workerCall
+) {
+  await SpecialPowers.pushPrefEnv({
+    set: [
+      ["privacy.resistFingerprinting", resistFingerprinting],
+      ["privacy.reduceTimerPrecision", reduceTimerPrecision],
+      [
+        "privacy.resistFingerprinting.reduceTimerPrecision.microseconds",
+        expectedPrecision * 1000,
+      ],
+      ["browser.tabs.remote.useCrossOriginOpenerPolicy", crossOriginIsolated],
+      ["browser.tabs.remote.useCrossOriginEmbedderPolicy", crossOriginIsolated],
+      ["browser.tabs.documentchannel", crossOriginIsolated],
+    ],
+  });
+
+  let win = await BrowserTestUtils.openNewBrowserWindow();
+  let tab = await BrowserTestUtils.openNewForegroundTab(
+    win.gBrowser,
+    `https://example.com/browser/browser/components/resistfingerprinting` +
+      `/test/browser/coop_header.sjs?crossOriginIsolated=${crossOriginIsolated}`
+  );
+
+  // No matter what we set the precision to, if we're in ResistFingerprinting mode
+  // we use the larger of the precision pref and the constant 100ms
+  if (resistFingerprinting) {
+    expectedPrecision = expectedPrecision < 100 ? 100 : expectedPrecision;
+  }
+  await SpecialPowers.spawn(
+    tab.linkedBrowser,
+    [
+      {
+        precision: expectedPrecision,
+        isRoundedFunc: isRounded.toString(),
+        workerCall,
+      },
+    ],
+    runTests
+  );
+
+  if (crossOriginIsolated) {
+    let remoteType = tab.linkedBrowser.remoteType;
+    ok(
+      remoteType.startsWith(E10SUtils.WEB_REMOTE_COOP_COEP_TYPE_PREFIX),
+      `${remoteType} expected to be coop+coep`
+    );
+  }
+
+  await BrowserTestUtils.closeWindow(win);
+};
+
+// ================================================================================================
+// ================================================================================================
+// This test case is mostly copy-and-paste from the forth test case in
+// browser_performanceAPI.js. The main difference is this test case verifies
+// performance API have more precsion when it's in cross-origin-isolated and
+// cross-origin-isolated doesn't affect RFP.
+let runWorkerTest = async function(data) {
+  let expectedPrecision = data.precision;
+  await new Promise(resolve => {
+    // eslint beleives that isrounded is available in this scope, but if you
+    // remove the assignment, you will see it is not
+    // eslint-disable-next-line
+    let isRounded = eval(data.isRoundedFunc);
+
+    let worker = new content.Worker(
+      "coop_header.sjs?crossOriginIsolated=true&worker=true"
+    );
+
+    worker.postMessage({
+      type: "runCmdAndGetResult",
+      cmd: `performance.timeOrigin`,
+    });
+
+    const expectedAllEntriesLength = data.workerCall == "runRPTests" ? 0 : 3;
+    const expectedResourceEntriesLength =
+      data.workerCall == "runRPTests" ? 0 : 2;
+    const expectedTestAndMarkEntriesLength =
+      data.workerCall == "runRPTests" ? 0 : 1;
+
+    worker.onmessage = function(e) {
+      if (e.data.type == "result") {
+        if (e.data.resultOf == "performance.timeOrigin") {
+          ok(
+            isRounded(e.data.result, expectedPrecision),
+            `In a worker, for reduceTimerPrecision, performance.timeOrigin is` +
+              ` not correctly rounded: ${e.data.result}`
+          );
+
+          worker.postMessage({
+            type: "runCmds",
+            cmds: [
+              `performance.mark("Test");`,
+              `performance.mark("Test-End");`,
+              `performance.measure("Test-Measure", "Test", "Test-End");`,
+            ],
+          });
+        } else if (e.data.resultOf == "entriesLength") {
+          is(
+            e.data.result,
+            expectedAllEntriesLength,
+            `In a worker, for reduceTimerPrecision: Incorrect number of ` +
+              `entries for performance.getEntries() for workers: ` +
+              `${e.data.result}`
+          );
+
+          if (data.workerCall == "runRTPTests") {
+            worker.postMessage({
+              type: "getResult",
+              resultOf: "startTimeAndDuration",
+              num: 0,
+            });
+          } else {
+            worker.postMessage({
+              type: "getResult",
+              resultOf: "getEntriesByTypeLength",
+            });
+          }
+        } else if (e.data.resultOf == "startTimeAndDuration") {
+          let index = e.data.result.index;
+          let startTime = e.data.result.startTime;
+          let duration = e.data.result.duration;
+          ok(
+            isRounded(startTime, expectedPrecision),
+            `In a worker, for reduceTimerPrecision(${expectedPrecision}, ` +
+              `performance.getEntries(${index}).startTime is not rounded: ` +
+              `${startTime}`
+          );
+          ok(
+            isRounded(duration, expectedPrecision),
+            `In a worker, for reduceTimerPrecision(${expectedPrecision}), ` +
+              `performance.getEntries(${index}).duration is not rounded: ` +
+              `${duration}`
+          );
+
+          if (index < 2) {
+            worker.postMessage({
+              type: "getResult",
+              resultOf: "startTimeAndDuration",
+              num: index + 1,
+            });
+          } else {
+            worker.postMessage({
+              type: "getResult",
+              resultOf: "getEntriesByTypeLength",
+            });
+          }
+        } else if (e.data.resultOf == "entriesByTypeLength") {
+          is(
+            e.data.result.markLength,
+            expectedResourceEntriesLength,
+            `In a worker, for reduceTimerPrecision: Incorrect number of ` +
+              `entries for performance.getEntriesByType() for workers: ` +
+              `${e.data.result.resourceLength}`
+          );
+          is(
+            e.data.result.testAndMarkLength,
+            expectedTestAndMarkEntriesLength,
+            `In a worker, for reduceTimerPrecision: Incorrect number of ` +
+              `entries for performance.getEntriesByName() for workers: ` +
+              `${e.data.result.testAndMarkLength}`
+          );
+
+          worker.terminate();
+          resolve();
+        }
+      } else {
+        ok(false, `Unknown message type got ${e.data.type}`);
+        worker.terminate();
+        resolve();
+      }
+    };
+  });
+};
+
+add_task(async function runTestsForWorker() {
+  // RFP
+  await setupTest(true, true, true, 100, runWorkerTest, "runRPTests");
+  await setupTest(true, false, true, 13, runWorkerTest, "runRPTests");
+  await setupTest(true, true, true, 0.13, runWorkerTest, "runRPTests");
+
+  // RTP
+  await setupTest(false, true, false, 0.13, runWorkerTest, "runRTPTests");
+  await setupTest(false, true, true, 0.005, runWorkerTest, "runRTPTests");
+});
+
+// ================================================================================================
+// ================================================================================================
+// This test case is mostly copy-and-paste from the test case for window in
+// test_reduce_time_precision.html. The main difference is this test case
+// verifies DOM API has more precsion when it's in cross-origin-isolated and
+// cross-origin-isolated doesn't affect RFP.
+add_task(async function runRTPTestDOM() {
+  let runTests = async function(data) {
+    let expectedPrecision = data.precision;
+    // eslint beleives that isrounded is available in this scope, but if you
+    // remove the assignment, you will see it is not
+    // eslint-disable-next-line
+    let isRounded = eval(data.isRoundedFunc);
+
+    // Prepare for test of AudioContext.currentTime
+    // eslint-disable-next-line
+    let audioContext = new content.AudioContext();
+
+    // Known ways to generate time stamps, in milliseconds
+    const timeStampCodes = [
+      "content.performance.now()",
+      "new content.Date().getTime()",
+      'new content.Event("").timeStamp',
+      'new content.File([], "").lastModified',
+    ];
+    // These are measured in seconds, so we need to scale them up
+    var timeStampCodesDOM = timeStampCodes.concat([
+      "audioContext.currentTime * 1000",
+    ]);
+
+    // Loop through each timeStampCode, evaluate it,
+    // and check if it is rounded
+    for (let timeStampCode of timeStampCodesDOM) {
+      // eslint-disable-next-line
+      let timeStamp = eval(timeStampCode);
+
+      // Audio Contexts increment in intervals of (minimum) 5.4ms, so we don't
+      // clamp/jitter if the timer precision is les than that.
+      // (Technically on MBPs they increment in intervals of 2.6 but this is
+      // non-standard and will eventually be changed. We don't cover this situation
+      // because we don't really support arbitrary Timer Precision, especially in
+      // the 2.6 - 5.4ms interval.)
+      if (timeStampCode.includes("audioContext") && expectedPrecision < 5.4) {
+        continue;
+      }
+
+      ok(
+        isRounded(timeStamp, expectedPrecision),
+        `Should be rounded to nearest ${expectedPrecision} ms; saw ${timeStamp}`
+      );
+    }
+  };
+
+  // RFP
+  await setupTest(true, true, true, 100, runTests);
+  await setupTest(true, false, true, 13, runTests);
+  await setupTest(true, false, true, 0.13, runTests);
+
+  // RTP
+  await setupTest(false, true, false, 0.13, runTests);
+  await setupTest(false, true, true, 0.005, runTests);
+});
+
+// ================================================================================================
+// ================================================================================================
+// This test case is mostly copy-and-paste from the test case for worker in
+// test_reduce_time_precision.html. The main difference is this test case
+// verifies DOM API has more precsion when it's in cross-origin-isolated and
+// cross-origin-isolated doesn't affect RFP.
+let runWorkerTest1 = async function(data) {
+  let expectedPrecision = data.precision;
+  await new Promise(resolve => {
+    // eslint beleives that isrounded is available in this scope, but if you
+    // remove the assignment, you will see it is not
+    // eslint-disable-next-line
+    let isRounded = eval(data.isRoundedFunc);
+
+    let worker = new content.Worker(
+      "coop_header.sjs?crossOriginIsolated=true&worker=true"
+    );
+
+    // Known ways to generate time stamps, in milliseconds
+    const timeStampCodes = [
+      "performance.now()",
+      "new Date().getTime()",
+      'new Event("").timeStamp',
+      'new File([], "").lastModified',
+    ];
+
+    let promises = [];
+    for (let timeStampCode of timeStampCodes) {
+      promises.push(
+        new Promise(res => {
+          worker.postMessage({
+            type: "runCmdAndGetResult",
+            cmd: timeStampCode,
+          });
+
+          worker.addEventListener("message", function(e) {
+            if (e.data.type == "result") {
+              if (e.data.resultOf == timeStampCode) {
+                ok(
+                  isRounded(e.data.result, expectedPrecision),
+                  `The result of ${e.data.resultOf} should be rounded to ` +
+                    ` nearest ${expectedPrecision} ms in workers; saw ` +
+                    `${e.data.result}`
+                );
+                worker.removeEventListener("message", this);
+                res();
+              }
+
+              return;
+            }
+
+            ok(false, `Unknown message type. Got ${e.data.type}`);
+            res();
+          });
+        })
+      );
+    }
+
+    Promise.all(promises).then(_ => {
+      worker.terminate();
+      resolve();
+    });
+  });
+};
+
+add_task(async function runRTPTestsForWorker() {
+  // RFP
+  await setupTest(true, true, true, 100, runWorkerTest1);
+  await setupTest(true, false, true, 13, runWorkerTest1);
+  await setupTest(true, false, true, 0.13, runWorkerTest1);
+
+  // RTP
+  await setupTest(false, true, false, 0.13, runWorkerTest1);
+  await setupTest(false, true, true, 0.005, runWorkerTest1);
+});
diff --git a/browser/components/resistfingerprinting/test/browser/coop_header.sjs b/browser/components/resistfingerprinting/test/browser/coop_header.sjs
new file mode 100644
index 000000000000..c02d9b8fd3c5
--- /dev/null
+++ b/browser/components/resistfingerprinting/test/browser/coop_header.sjs
@@ -0,0 +1,77 @@
+const HTML_DATA = `
+  <!DOCTYPE HTML>
+  <html>
+  <head>
+  <title>Dummy test page</title>
+  <meta http-equiv="Content-Type" content="text/html;charset=utf-8"></meta>
+  </head>
+  <body>
+  <p>Dummy test page</p>
+  </body>
+  </html>
+  `;
+
+const WORKER = `
+  self.onmessage = function(e) {
+    if (e.data.type == "runCmdAndGetResult") {
+      let result = eval(e.data.cmd);
+
+      postMessage({ type: "result", resultOf: e.data.cmd, result: result });
+      return;
+    } else if (e.data.type == "runCmds") {
+       for (let cmd of e.data.cmds) {
+         eval(cmd);
+       }
+
+       postMessage({ type: "result", resultOf: "entriesLength", result: performance.getEntries().length });
+       return;
+    } else if (e.data.type == "getResult") {
+      if (e.data.resultOf == "startTimeAndDuration") {
+         postMessage({ type: "result",
+                       resultOf: "startTimeAndDuration",
+                       result: {
+                         index: e.data.num,
+                         startTime: performance.getEntries()[e.data.num].startTime,
+                         duration: performance.getEntries()[e.data.num].duration,
+                       }
+         });
+         return;
+     } else if (e.data.resultOf == "getEntriesByTypeLength") {
+        postMessage({
+                      type: "result",
+                      resultOf: "entriesByTypeLength",
+                      result: {
+                      markLength: performance.getEntriesByType("mark").length,
+                      resourceLength: performance.getEntriesByType("resource").length,
+                      testAndMarkLength: performance.getEntriesByName("Test", "mark").length,
+                    }
+        });
+        return;
+      }
+    }
+
+    postMessage({ type: "unexpectedMessageType", data: e.data.type });
+  };
+  `;
+
+function handleRequest(request, response)
+{
+  Components.utils.importGlobalProperties(["URLSearchParams"]);
+  let query = new URLSearchParams(request.queryString);
+
+  if (query.get("crossOriginIsolated") === "true") {
+    response.setHeader("Cross-Origin-Opener-Policy", "same-origin", false);
+    response.setHeader("Cross-Origin-Embedder-Policy", "require-corp", false);
+  }
+
+  if (query.get("worker")) {
+    response.setHeader("Cache-Control", "no-cache", false);
+    response.setHeader("Content-Type", "application/javascript");
+    response.write(WORKER);
+    return;
+  }
+
+  response.setHeader("Cache-Control", "no-cache", false);
+  response.setHeader("Content-Type", "text/html", false);
+  response.write(HTML_DATA);
+}