diff --git a/dom/security/test/cors/file_CrossSiteXHR_cache_server.sjs b/dom/security/test/cors/file_CrossSiteXHR_cache_server.sjs
index ef6c7230493d..431559d50409 100644
--- a/dom/security/test/cors/file_CrossSiteXHR_cache_server.sjs
+++ b/dom/security/test/cors/file_CrossSiteXHR_cache_server.sjs
@@ -1,5 +1,4 @@
 Cu.import("resource://gre/modules/Services.jsm");
-Services.prefs.setBoolPref("security.allow_eval_with_system_principal", true);
 
 function handleRequest(request, response)
 {
@@ -25,7 +24,7 @@ function handleRequest(request, response)
   // Send response
 
   secData =
-    eval(getState("test/dom/security/test_CrossSiteXHR_cache:secData"));
+    JSON.parse(getState("test/dom/security/test_CrossSiteXHR_cache:secData"));
 
   if (secData.allowOrigin)
     response.setHeader("Access-Control-Allow-Origin", secData.allowOrigin);
diff --git a/dom/security/test/cors/file_CrossSiteXHR_inner.html b/dom/security/test/cors/file_CrossSiteXHR_inner.html
index ae3b58b04a0b..d3e84213629d 100644
--- a/dom/security/test/cors/file_CrossSiteXHR_inner.html
+++ b/dom/security/test/cors/file_CrossSiteXHR_inner.html
@@ -16,7 +16,7 @@ window.addEventListener("message", function(e) {
 
   sendData = null;
 
-  req = eval(e.data);
+  req = JSON.parse(e.data);
   var res = {
     didFail: false,
     events: [],
@@ -110,7 +110,7 @@ window.addEventListener("message", function(e) {
 });
 
 function post(e, res) {
-  e.source.postMessage(res.toSource(), "http://mochi.test:8888");
+  e.source.postMessage(JSON.stringify(res), "http://mochi.test:8888");
 }
 
 </script>
diff --git a/dom/security/test/cors/file_CrossSiteXHR_inner_data.sjs b/dom/security/test/cors/file_CrossSiteXHR_inner_data.sjs
index 2908921e2bdf..50356239f4d1 100644
--- a/dom/security/test/cors/file_CrossSiteXHR_inner_data.sjs
+++ b/dom/security/test/cors/file_CrossSiteXHR_inner_data.sjs
@@ -6,7 +6,7 @@ window.addEventListener("message", function(e) {\n\
 \n\
   sendData = null;\n\
 \n\
-  req = eval(e.data);\n\
+  req = JSON.parse(e.data);\n\
   var res = {\n\
     didFail: false,\n\
     events: [],\n\
@@ -81,7 +81,7 @@ window.addEventListener("message", function(e) {\n\
 }, false);\n\
 \n\
 function post(e, res) {\n\
-  e.source.postMessage(res.toSource(), "*");\n\
+  e.source.postMessage(JSON.stringify(res), "*");\n\
 }\n\
 function trimString(stringValue) {\n\
   return stringValue.replace("/^\s+|\s+$/g","");\n\
diff --git a/dom/security/test/cors/file_CrossSiteXHR_server.sjs b/dom/security/test/cors/file_CrossSiteXHR_server.sjs
index 68f6913b5c4e..143561a25ae6 100644
--- a/dom/security/test/cors/file_CrossSiteXHR_server.sjs
+++ b/dom/security/test/cors/file_CrossSiteXHR_server.sjs
@@ -25,7 +25,7 @@ function handleRequest(request, response)
 
   if (query.hop) {
     query.hop = parseInt(query.hop, 10);
-    hops = eval(query.hops);
+    hops = JSON.parse(query.hops);
     var curHop = hops[query.hop - 1];
     query.allowOrigin = curHop.allowOrigin;
     query.allowHeaders = curHop.allowHeaders;
@@ -50,7 +50,7 @@ function handleRequest(request, response)
   }
 
   if (!isPreflight && "headers" in query) {
-    headers = eval(query.headers);
+    headers = JSON.parse(query.headers);
     for(headerName in headers) {
       // Content-Type is changed if there was a body 
       if (!(headerName == "Content-Type" && body) &&
@@ -142,7 +142,7 @@ function handleRequest(request, response)
   }
   else {
     if (query.responseHeaders) {
-      let responseHeaders = eval(query.responseHeaders);
+      let responseHeaders = JSON.parse(query.responseHeaders);
       for (let responseHeader in responseHeaders) {
         response.setHeader(responseHeader, responseHeaders[responseHeader]);
       }
diff --git a/dom/security/test/cors/file_bug1456721.html b/dom/security/test/cors/file_bug1456721.html
index 429bacf5633c..20ef00d06d1e 100644
--- a/dom/security/test/cors/file_bug1456721.html
+++ b/dom/security/test/cors/file_bug1456721.html
@@ -61,9 +61,9 @@ function* runTest() {
       method: test.method
     };
 
-    loaderWindow.postMessage(req.toSource(), origin);
+    loaderWindow.postMessage(JSON.stringify(req), origin);
     number_of_tests++;
-    eval(yield);
+    yield;
   }
 }
 
diff --git a/dom/security/test/cors/file_cors_logging_test.html b/dom/security/test/cors/file_cors_logging_test.html
index 367cc3d63c5b..d29f93cf9c38 100644
--- a/dom/security/test/cors/file_cors_logging_test.html
+++ b/dom/security/test/cors/file_cors_logging_test.html
@@ -663,7 +663,7 @@ function* runTest() {
                  "application/x-www-form-urlencoded"]
                    .includes(test.headers[name].toLowerCase()));
       }
-      req.url += "&headers=" + escape(test.headers.toSource());
+      req.url += "&headers=" + escape(JSON.stringify(test.headers));
       reqHeaders =
         escape(Object.keys(test.headers)
                .filter(isUnsafeHeader)
@@ -685,14 +685,14 @@ function* runTest() {
     if (test.preflightStatus)
       req.url += "&preflightStatus=" + test.preflightStatus;
     if (test.responseHeaders)
-      req.url += "&responseHeaders=" + escape(test.responseHeaders.toSource());
+      req.url += "&responseHeaders=" + escape(JSON.stringify(test.responseHeaders));
     if (test.exposeHeaders)
       req.url += "&exposeHeaders=" + escape(test.exposeHeaders);
     if (test.preflightBody)
       req.url += "&preflightBody=" + escape(test.preflightBody);
 
-    loaderWindow.postMessage(req.toSource(), origin);
-    res = eval(yield);
+    loaderWindow.postMessage(JSON.stringify(req), origin);
+    res = JSON.parse(yield);
   }
 
   // Test cookie behavior
@@ -795,9 +795,9 @@ function* runTest() {
     if ("allowMethods" in test)
       req.url += "&allowMethods=" + escape(test.allowMethods);
 
-    loaderWindow.postMessage(req.toSource(), origin);
+    loaderWindow.postMessage(JSON.stringify(req), origin);
 
-    res = eval(yield);
+    res = JSON.parse(yield);
   }
 
   // Make sure to clear cookies to avoid affecting other tests
@@ -1285,7 +1285,7 @@ function* runTest() {
   for (test of tests) {
     req = {
       url: test.hops[0].server + basePath + "hop=1&hops=" +
-           escape(test.hops.toSource()),
+           escape(JSON.stringify(test.hops)),
       method: test.method,
       headers: test.headers,
       body: test.body,
@@ -1297,9 +1297,9 @@ function* runTest() {
         req.url += "&body=" + escape(test.body);
     }
 
-    loaderWindow.postMessage(req.toSource(), origin);
+    loaderWindow.postMessage(JSON.stringify(req), origin);
 
-    res = eval(yield);
+    res = JSON.parse(yield);
   }
 
   document.location.href += "#finished";
diff --git a/dom/security/test/cors/test_CrossSiteXHR.html b/dom/security/test/cors/test_CrossSiteXHR.html
index d41c1d2ae583..93aff72cc71f 100644
--- a/dom/security/test/cors/test_CrossSiteXHR.html
+++ b/dom/security/test/cors/test_CrossSiteXHR.html
@@ -25,8 +25,7 @@ var gen;
 function initTest() {
   SimpleTest.waitForExplicitFinish();
   // Allow all cookies, then do the actual test initialization
-  SpecialPowers.pushPrefEnv({"set": [["network.cookie.cookieBehavior", 0],
-  																	 ["security.allow_eval_with_system_principal", true]]},
+  SpecialPowers.pushPrefEnv({"set": [["network.cookie.cookieBehavior", 0]]},
   																	  initTestCallback);
 }
 
@@ -670,7 +669,7 @@ function* runTest() {
                  "application/x-www-form-urlencoded"]
                    .includes(test.headers[name].toLowerCase()));
       }
-      req.url += "&headers=" + escape(test.headers.toSource());
+      req.url += "&headers=" + escape(JSON.stringify(test.headers));
       reqHeaders =
         escape(Object.keys(test.headers)
                .filter(isUnsafeHeader)
@@ -692,86 +691,86 @@ function* runTest() {
     if (test.preflightStatus)
       req.url += "&preflightStatus=" + test.preflightStatus;
     if (test.responseHeaders)
-      req.url += "&responseHeaders=" + escape(test.responseHeaders.toSource());
+      req.url += "&responseHeaders=" + escape(JSON.stringify(test.responseHeaders));
     if (test.exposeHeaders)
       req.url += "&exposeHeaders=" + escape(test.exposeHeaders);
     if (test.preflightBody)
       req.url += "&preflightBody=" + escape(test.preflightBody);
 
-    loaderWindow.postMessage(req.toSource(), origin);
-    res = eval(yield);
+    loaderWindow.postMessage(JSON.stringify(req), origin);
+    res = JSON.parse(yield);
 
     if (test.pass) {
       is(res.didFail, false,
-        "shouldn't have failed in test for " + test.toSource());
+        "shouldn't have failed in test for " + JSON.stringify(test));
       if (test.status) {
-        is(res.status, test.status, "wrong status in test for " + test.toSource());
-        is(res.statusText, test.statusMessage, "wrong status text for " + test.toSource());
+        is(res.status, test.status, "wrong status in test for " + JSON.stringify(test));
+        is(res.statusText, test.statusMessage, "wrong status text for " + JSON.stringify(test));
       }
       else {
-        is(res.status, 200, "wrong status in test for " + test.toSource());
-        is(res.statusText, "OK", "wrong status text for " + test.toSource());
+        is(res.status, 200, "wrong status in test for " + JSON.stringify(test));
+        is(res.statusText, "OK", "wrong status text for " + JSON.stringify(test));
       }
       if (test.method !== "HEAD") {
         is(res.responseXML, "<res>hello pass</res>",
-           "wrong responseXML in test for " + test.toSource());
+           "wrong responseXML in test for " + JSON.stringify(test));
         is(res.responseText, "<res>hello pass</res>\n",
-           "wrong responseText in test for " + test.toSource());
+           "wrong responseText in test for " + JSON.stringify(test));
         is(res.events.join(","),
            "opening,rs1,sending,loadstart,rs2,rs3,rs4,load,loadend",
-           "wrong responseText in test for " + test.toSource());
+           "wrong responseText in test for " + JSON.stringify(test));
       }
       else {
         is(res.responseXML, null,
-           "wrong responseXML in test for " + test.toSource());
+           "wrong responseXML in test for " + JSON.stringify(test));
         is(res.responseText, "",
-           "wrong responseText in test for " + test.toSource());
+           "wrong responseText in test for " + JSON.stringify(test));
         is(res.events.join(","),
            "opening,rs1,sending,loadstart,rs2,rs4,load,loadend",
-           "wrong responseText in test for " + test.toSource());
+           "wrong responseText in test for " + JSON.stringify(test));
       }
       if (test.responseHeaders) {
         for (header in test.responseHeaders) {
           if (!test.expectedResponseHeaders.includes(header)) {
             is(res.responseHeaders[header], null,
                "|xhr.getResponseHeader()|wrong response header (" + header + ") in test for " +
-               test.toSource());
+               JSON.stringify(test));
             is(res.allResponseHeaders[header], undefined,
               "|xhr.getAllResponseHeaderss()|wrong response header (" + header + ") in test for " +
-              test.toSource());
+              JSON.stringify(test));
           }
           else {
             is(res.responseHeaders[header], test.responseHeaders[header],
                "|xhr.getResponseHeader()|wrong response header (" + header + ") in test for " +
-               test.toSource());
+               JSON.stringify(test));
             is(res.allResponseHeaders[header.toLowerCase()], test.responseHeaders[header],
               "|xhr.getAllResponseHeaderss()|wrong response header (" + header + ") in test for " +
-              test.toSource());
+              JSON.stringify(test));
           }
         }
       }
     }
     else {
       is(res.didFail, true,
-        "should have failed in test for " + test.toSource());
-      is(res.status, 0, "wrong status in test for " + test.toSource());
-      is(res.statusText, "", "wrong status text for " + test.toSource());
+        "should have failed in test for " + JSON.stringify(test));
+      is(res.status, 0, "wrong status in test for " + JSON.stringify(test));
+      is(res.statusText, "", "wrong status text for " + JSON.stringify(test));
       is(res.responseXML, null,
-         "wrong responseXML in test for " + test.toSource());
+         "wrong responseXML in test for " + JSON.stringify(test));
       is(res.responseText, "",
-         "wrong responseText in test for " + test.toSource());
+         "wrong responseText in test for " + JSON.stringify(test));
       if (!res.sendThrew) {
         is(res.events.join(","),
            "opening,rs1,sending,loadstart,rs4,error,loadend",
-           "wrong events in test for " + test.toSource());
+           "wrong events in test for " + JSON.stringify(test));
       }
       is(res.progressEvents, 0,
-         "wrong events in test for " + test.toSource());
+         "wrong events in test for " + JSON.stringify(test));
       if (test.responseHeaders) {
         for (header in test.responseHeaders) {
           is(res.responseHeaders[header], null,
              "wrong response header (" + header + ") in test for " +
-             test.toSource());
+             JSON.stringify(test));
         }
       }
     }
@@ -877,36 +876,36 @@ function* runTest() {
     if ("allowMethods" in test)
       req.url += "&allowMethods=" + escape(test.allowMethods);
 
-    loaderWindow.postMessage(req.toSource(), origin);
+    loaderWindow.postMessage(JSON.stringify(req), origin);
 
-    res = eval(yield);
+    res = JSON.parse(yield);
     if (test.pass) {
       is(res.didFail, false,
-        "shouldn't have failed in test for " + test.toSource());
-      is(res.status, 200, "wrong status in test for " + test.toSource());
-      is(res.statusText, "OK", "wrong status text for " + test.toSource());
+        "shouldn't have failed in test for " + JSON.stringify(test));
+      is(res.status, 200, "wrong status in test for " + JSON.stringify(test));
+      is(res.statusText, "OK", "wrong status text for " + JSON.stringify(test));
       is(res.responseXML, "<res>hello pass</res>",
-         "wrong responseXML in test for " + test.toSource());
+         "wrong responseXML in test for " + JSON.stringify(test));
       is(res.responseText, "<res>hello pass</res>\n",
-         "wrong responseText in test for " + test.toSource());
+         "wrong responseText in test for " + JSON.stringify(test));
       is(res.events.join(","),
          "opening,rs1,sending,loadstart,rs2,rs3,rs4,load,loadend",
-         "wrong responseText in test for " + test.toSource());
+         "wrong responseText in test for " + JSON.stringify(test));
     }
     else {
       is(res.didFail, true,
-        "should have failed in test for " + test.toSource());
-      is(res.status, 0, "wrong status in test for " + test.toSource());
-      is(res.statusText, "", "wrong status text for " + test.toSource());
+        "should have failed in test for " + JSON.stringify(test));
+      is(res.status, 0, "wrong status in test for " + JSON.stringify(test));
+      is(res.statusText, "", "wrong status text for " + JSON.stringify(test));
       is(res.responseXML, null,
-         "wrong responseXML in test for " + test.toSource());
+         "wrong responseXML in test for " + JSON.stringify(test));
       is(res.responseText, "",
-         "wrong responseText in test for " + test.toSource());
+         "wrong responseText in test for " + JSON.stringify(test));
       is(res.events.join(","),
          "opening,rs1,sending,loadstart,rs4,error,loadend",
-         "wrong events in test for " + test.toSource());
+         "wrong events in test for " + JSON.stringify(test));
       is(res.progressEvents, 0,
-         "wrong events in test for " + test.toSource());
+         "wrong events in test for " + JSON.stringify(test));
     }
   }
 
@@ -1431,7 +1430,7 @@ function* runTest() {
   for (test of tests) {
     req = {
       url: test.hops[0].server + basePath + "hop=1&hops=" +
-           escape(test.hops.toSource()),
+           escape(JSON.stringify(test.hops)),
       method: test.method,
       headers: test.headers,
       body: test.body,
@@ -1443,36 +1442,36 @@ function* runTest() {
         req.url += "&body=" + escape(test.body);
     }
 
-    loaderWindow.postMessage(req.toSource(), origin);
+    loaderWindow.postMessage(JSON.stringify(req), origin);
 
-    res = eval(yield);
+    res = JSON.parse(yield);
     if (test.pass) {
       is(res.didFail, false,
-        "shouldn't have failed in test for " + test.toSource());
-      is(res.status, 200, "wrong status in test for " + test.toSource());
-      is(res.statusText, "OK", "wrong status text for " + test.toSource());
+        "shouldn't have failed in test for " + JSON.stringify(test));
+      is(res.status, 200, "wrong status in test for " + JSON.stringify(test));
+      is(res.statusText, "OK", "wrong status text for " + JSON.stringify(test));
       is(res.responseXML, "<res>hello pass</res>",
-         "wrong responseXML in test for " + test.toSource());
+         "wrong responseXML in test for " + JSON.stringify(test));
       is(res.responseText, "<res>hello pass</res>\n",
-         "wrong responseText in test for " + test.toSource());
+         "wrong responseText in test for " + JSON.stringify(test));
       is(res.events.join(","),
          "opening,rs1,sending,loadstart,rs2,rs3,rs4,load,loadend",
-         "wrong responseText in test for " + test.toSource());
+         "wrong responseText in test for " + JSON.stringify(test));
     }
     else {
       is(res.didFail, true,
-        "should have failed in test for " + test.toSource());
-      is(res.status, 0, "wrong status in test for " + test.toSource());
-      is(res.statusText, "", "wrong status text for " + test.toSource());
+        "should have failed in test for " + JSON.stringify(test));
+      is(res.status, 0, "wrong status in test for " + JSON.stringify(test));
+      is(res.statusText, "", "wrong status text for " + JSON.stringify(test));
       is(res.responseXML, null,
-         "wrong responseXML in test for " + test.toSource());
+         "wrong responseXML in test for " + JSON.stringify(test));
       is(res.responseText, "",
-         "wrong responseText in test for " + test.toSource());
+         "wrong responseText in test for " + JSON.stringify(test));
       is(res.events.join(","),
          "opening,rs1,sending,loadstart,rs4,error,loadend",
-         "wrong events in test for " + test.toSource());
+         "wrong events in test for " + JSON.stringify(test));
       is(res.progressEvents, 0,
-         "wrong progressevents in test for " + test.toSource());
+         "wrong progressevents in test for " + JSON.stringify(test));
     }
   }
 
diff --git a/dom/security/test/cors/test_CrossSiteXHR_cache.html b/dom/security/test/cors/test_CrossSiteXHR_cache.html
index 557d8a5cc44a..437c9d9d8b7f 100644
--- a/dom/security/test/cors/test_CrossSiteXHR_cache.html
+++ b/dom/security/test/cors/test_CrossSiteXHR_cache.html
@@ -20,9 +20,6 @@ let gen;
 SimpleTest.waitForExplicitFinish();
 SimpleTest.requestFlakyTimeout("This test needs to generate artificial pauses, hence it uses timeouts.  There is no way around it, unfortunately. :(");
 
-SpecialPowers.pushPrefEnv({"set": [["security.allow_eval_with_system_principal",
-                                     true]]});
-
 window.addEventListener("message", function(e) {
   gen.next(e.data);
 });
@@ -541,16 +538,16 @@ function* runTest() {
             cacheTime: test.cacheTime,
             withCred: test.withCred };
     xhr = new XMLHttpRequest();
-    xhr.open("POST", setStateURL + escape(sec.toSource()), true);
+    xhr.open("POST", setStateURL + escape(JSON.stringify(sec)), true);
     xhr.onloadend = function() { gen.next(); }
     xhr.send();
     yield undefined;
 
-    loaderWindow.postMessage(req.toSource(), origin);
+    loaderWindow.postMessage(JSON.stringify(req), origin);
 
-    res = eval(yield);
+    res = JSON.parse(yield);
 
-    testName = test.toSource() + " (index " + tests.indexOf(test) + ")";
+    testName = JSON.stringify(test) + " (index " + tests.indexOf(test) + ")";
 
     if (test.pass) {
       is(res.didFail, false,
diff --git a/dom/security/test/cors/test_CrossSiteXHR_origin.html b/dom/security/test/cors/test_CrossSiteXHR_origin.html
index 218ccccee74f..31cc5809d595 100644
--- a/dom/security/test/cors/test_CrossSiteXHR_origin.html
+++ b/dom/security/test/cors/test_CrossSiteXHR_origin.html
@@ -118,9 +118,9 @@ function* runTest() {
              "&origin=" + escape(origin),
         method: "GET",
       };
-      loaderWindow.postMessage(req.toSource(), isNullOrigin ? "*" : origin);
+      loaderWindow.postMessage(JSON.stringify(req), isNullOrigin ? "*" : origin);
 
-      res = eval(yield);
+      res = JSON.parse(yield);
       is(res.didFail, false, "shouldn't have failed for " + allowOrigin);
       is(res.status, 200, "wrong status for " + allowOrigin);
       is(res.statusText, "OK", "wrong status text for " + allowOrigin);
@@ -139,9 +139,9 @@ function* runTest() {
         url: baseURL + "allowOrigin=" + escape(allowOrigin),
         method: "GET",
       };
-      loaderWindow.postMessage(req.toSource(), isNullOrigin ? "*" : origin);
+      loaderWindow.postMessage(JSON.stringify(req), isNullOrigin ? "*" : origin);
 
-      res = eval(yield);
+      res = JSON.parse(yield);
       is(res.didFail, true, "should have failed for " + allowOrigin);
       is(res.responseText, "", "should have no text for " + allowOrigin);
       is(res.status, 0, "should have no status for " + allowOrigin);
diff --git a/dom/tests/mochitest/fetch/test_fetch_cors.js b/dom/tests/mochitest/fetch/test_fetch_cors.js
index f57ac34d0bc4..50d6cca7fbee 100644
--- a/dom/tests/mochitest/fetch/test_fetch_cors.js
+++ b/dom/tests/mochitest/fetch/test_fetch_cors.js
@@ -164,14 +164,14 @@ function testSameOriginCredentials() {
   }
 
   function testResponse(res, test) {
-    ok(test.pass, "Expected test to pass " + test.toSource());
-    is(res.status, 200, "wrong status in test for " + test.toSource());
-    is(res.statusText, "OK", "wrong status text for " + test.toSource());
+    ok(test.pass, "Expected test to pass " + JSON.stringify(test));
+    is(res.status, 200, "wrong status in test for " + JSON.stringify(test));
+    is(res.statusText, "OK", "wrong status text for " + JSON.stringify(test));
     return res.text().then(function(v) {
       is(
         v,
         "<res>hello pass</res>\n",
-        "wrong text in test for " + test.toSource()
+        "wrong text in test for " + JSON.stringify(test)
       );
     });
   }
@@ -191,8 +191,8 @@ function testSameOriginCredentials() {
         });
       },
       function(e) {
-        ok(!test.pass, "Expected test to fail " + test.toSource());
-        ok(e instanceof TypeError, "Test should fail " + test.toSource());
+        ok(!test.pass, "Expected test to fail " + JSON.stringify(test));
+        ok(e instanceof TypeError, "Test should fail " + JSON.stringify(test));
         if (i < tests.length - 1) {
           runATest(tests, i + 1);
         } else {
@@ -804,7 +804,7 @@ function testModeCors() {
             ].includes(test.headers[name].toLowerCase()))
         );
       }
-      req.url += "&headers=" + escape(test.headers.toSource());
+      req.url += "&headers=" + escape(JSON.stringify(test.headers));
       reqHeaders = escape(
         Object.keys(test.headers)
           .filter(isUnsafeHeader)
@@ -831,7 +831,8 @@ function testModeCors() {
       req.url += "&preflightStatus=" + test.preflightStatus;
     }
     if (test.responseHeaders) {
-      req.url += "&responseHeaders=" + escape(test.responseHeaders.toSource());
+      req.url +=
+        "&responseHeaders=" + escape(JSON.stringify(test.responseHeaders));
     }
     if (test.exposeHeaders) {
       req.url += "&exposeHeaders=" + escape(test.exposeHeaders);
@@ -856,28 +857,28 @@ function testModeCors() {
             return fetch(request);
           })
           .then(function(res) {
-            ok(test.pass, "Expected test to pass for " + test.toSource());
+            ok(test.pass, "Expected test to pass for " + JSON.stringify(test));
             if (test.status) {
               is(
                 res.status,
                 test.status,
-                "wrong status in test for " + test.toSource()
+                "wrong status in test for " + JSON.stringify(test)
               );
               is(
                 res.statusText,
                 test.statusMessage,
-                "wrong status text for " + test.toSource()
+                "wrong status text for " + JSON.stringify(test)
               );
             } else {
               is(
                 res.status,
                 200,
-                "wrong status in test for " + test.toSource()
+                "wrong status in test for " + JSON.stringify(test)
               );
               is(
                 res.statusText,
                 "OK",
-                "wrong status text for " + test.toSource()
+                "wrong status text for " + JSON.stringify(test)
               );
             }
             if (test.responseHeaders) {
@@ -889,7 +890,7 @@ function testModeCors() {
                     "|Headers.has()|wrong response header (" +
                       header +
                       ") in test for " +
-                      test.toSource()
+                      JSON.stringify(test)
                   );
                 } else {
                   is(
@@ -898,7 +899,7 @@ function testModeCors() {
                     "|Headers.get()|wrong response header (" +
                       header +
                       ") in test for " +
-                      test.toSource()
+                      JSON.stringify(test)
                   );
                 }
               }
@@ -911,21 +912,21 @@ function testModeCors() {
               is(
                 v,
                 "<res>hello pass</res>\n",
-                "wrong responseText in test for " + test.toSource()
+                "wrong responseText in test for " + JSON.stringify(test)
               );
             } else {
               is(
                 v,
                 "",
-                "wrong responseText in HEAD test for " + test.toSource()
+                "wrong responseText in HEAD test for " + JSON.stringify(test)
               );
             }
           })
           .catch(function(e) {
-            ok(!test.pass, "Expected test failure for " + test.toSource());
+            ok(!test.pass, "Expected test failure for " + JSON.stringify(test));
             ok(
               e instanceof TypeError,
-              "Exception should be TypeError for " + test.toSource()
+              "Exception should be TypeError for " + JSON.stringify(test)
             );
           });
       })(test)
@@ -1112,7 +1113,7 @@ function testCrossOriginCredentials() {
         test.hops[0].server +
         corsServerPath +
         "hop=1&hops=" +
-        escape(test.hops.toSource());
+        escape(JSON.stringify(test.hops));
     } else {
       url = baseURL + "allowOrigin=" + escape(test.origin || origin);
     }
@@ -1152,14 +1153,14 @@ function testCrossOriginCredentials() {
   }
 
   function testResponse(res, test) {
-    ok(test.pass, "Expected test to pass for " + test.toSource());
-    is(res.status, 200, "wrong status in test for " + test.toSource());
-    is(res.statusText, "OK", "wrong status text for " + test.toSource());
+    ok(test.pass, "Expected test to pass for " + JSON.stringify(test));
+    is(res.status, 200, "wrong status in test for " + JSON.stringify(test));
+    is(res.statusText, "OK", "wrong status text for " + JSON.stringify(test));
     return res.text().then(function(v) {
       is(
         v,
         "<res>hello pass</res>\n",
-        "wrong text in test for " + test.toSource()
+        "wrong text in test for " + JSON.stringify(test)
       );
     });
   }
@@ -1178,10 +1179,10 @@ function testCrossOriginCredentials() {
         });
       },
       function(e) {
-        ok(!test.pass, "Expected test failure for " + test.toSource());
+        ok(!test.pass, "Expected test failure for " + JSON.stringify(test));
         ok(
           e instanceof TypeError,
-          "Exception should be TypeError for " + test.toSource()
+          "Exception should be TypeError for " + JSON.stringify(test)
         );
         if (i < tests.length - 1) {
           runATest(tests, i + 1);
@@ -1270,7 +1271,7 @@ function testModeNoCorsCredentials() {
   }
 
   function testResponse(res, test) {
-    is(res.type, "opaque", "wrong response type for " + test.toSource());
+    is(res.type, "opaque", "wrong response type for " + JSON.stringify(test));
 
     // Get unfiltered response
     var chromeResponse = SpecialPowers.wrap(res);
@@ -1280,14 +1281,14 @@ function testModeNoCorsCredentials() {
     is(
       unfiltered.status,
       status,
-      "wrong status in test for " + test.toSource()
+      "wrong status in test for " + JSON.stringify(test)
     );
     return unfiltered.text().then(function(v) {
       if (status === 200) {
         is(
           v,
           "<res>hello pass</res>\n",
-          "wrong text in test for " + test.toSource()
+          "wrong text in test for " + JSON.stringify(test)
         );
       }
     });
@@ -1312,8 +1313,8 @@ function testModeNoCorsCredentials() {
         });
       },
       function(e) {
-        ok(!test.pass, "Expected test to fail " + test.toSource());
-        ok(e instanceof TypeError, "Test should fail " + test.toSource());
+        ok(!test.pass, "Expected test to fail " + JSON.stringify(test));
+        ok(e instanceof TypeError, "Test should fail " + JSON.stringify(test));
         if (i < tests.length - 1) {
           runATest(tests, i + 1);
         } else {
@@ -1619,14 +1620,14 @@ function testCORSRedirects() {
         test.hops[0].server +
         corsServerPath +
         "hop=1&hops=" +
-        escape(test.hops.toSource()),
+        escape(JSON.stringify(test.hops)),
       method: test.method,
       headers: test.headers,
       body: test.body,
     };
 
     if (test.headers) {
-      req.url += "&headers=" + escape(test.headers.toSource());
+      req.url += "&headers=" + escape(JSON.stringify(test.headers));
     }
 
     if (test.pass) {
@@ -1644,14 +1645,22 @@ function testCORSRedirects() {
       (function(request, test) {
         return fetch(request).then(
           function(res) {
-            ok(test.pass, "Expected test to pass for " + test.toSource());
-            is(res.status, 200, "wrong status in test for " + test.toSource());
+            ok(test.pass, "Expected test to pass for " + JSON.stringify(test));
+            is(
+              res.status,
+              200,
+              "wrong status in test for " + JSON.stringify(test)
+            );
             is(
               res.statusText,
               "OK",
-              "wrong status text for " + test.toSource()
+              "wrong status text for " + JSON.stringify(test)
+            );
+            is(
+              res.type,
+              "cors",
+              "wrong response type for " + JSON.stringify(test)
             );
-            is(res.type, "cors", "wrong response type for " + test.toSource());
             var reqHost = new URL(req.url).host;
             // If there is a service worker present, the redirections will be
             // transparent, assuming that the original request is to the current
@@ -1675,15 +1684,15 @@ function testCORSRedirects() {
               is(
                 v,
                 "<res>hello pass</res>\n",
-                "wrong responseText in test for " + test.toSource()
+                "wrong responseText in test for " + JSON.stringify(test)
               );
             });
           },
           function(e) {
-            ok(!test.pass, "Expected test failure for " + test.toSource());
+            ok(!test.pass, "Expected test failure for " + JSON.stringify(test));
             ok(
               e instanceof TypeError,
-              "Exception should be TypeError for " + test.toSource()
+              "Exception should be TypeError for " + JSON.stringify(test)
             );
           }
         );
@@ -1740,14 +1749,14 @@ function testNoCORSRedirects() {
         test.hops[0].server +
         corsServerPath +
         "hop=1&hops=" +
-        escape(test.hops.toSource()),
+        escape(JSON.stringify(test.hops)),
       method: test.method,
       headers: test.headers,
       body: test.body,
     };
 
     if (test.headers) {
-      req.url += "&headers=" + escape(test.headers.toSource());
+      req.url += "&headers=" + escape(JSON.stringify(test.headers));
     }
 
     if (test.pass) {
@@ -1773,31 +1782,45 @@ function testNoCORSRedirects() {
           })
           .then(
             function(res) {
-              ok(test.pass, "Expected test to pass for " + test.toSource());
+              ok(
+                test.pass,
+                "Expected test to pass for " + JSON.stringify(test)
+              );
               // All requests are cross-origin no-cors, we should always have
               // an opaque response here.  All values on the opaque response
               // should be hidden.
               is(
                 res.type,
                 "opaque",
-                "wrong response type for " + test.toSource()
+                "wrong response type for " + JSON.stringify(test)
+              );
+              is(
+                res.status,
+                0,
+                "wrong status in test for " + JSON.stringify(test)
               );
-              is(res.status, 0, "wrong status in test for " + test.toSource());
               is(
                 res.statusText,
                 "",
-                "wrong status text for " + test.toSource()
+                "wrong status text for " + JSON.stringify(test)
               );
-              is(res.url, "", "wrong response url for " + test.toSource());
+              is(res.url, "", "wrong response url for " + JSON.stringify(test));
               return res.text().then(function(v) {
-                is(v, "", "wrong responseText in test for " + test.toSource());
+                is(
+                  v,
+                  "",
+                  "wrong responseText in test for " + JSON.stringify(test)
+                );
               });
             },
             function(e) {
-              ok(!test.pass, "Expected test failure for " + test.toSource());
+              ok(
+                !test.pass,
+                "Expected test failure for " + JSON.stringify(test)
+              );
               ok(
                 e instanceof TypeError,
-                "Exception should be TypeError for " + test.toSource()
+                "Exception should be TypeError for " + JSON.stringify(test)
               );
             }
           );
@@ -1820,7 +1843,7 @@ function testReferrer() {
     Referer: referrer,
   };
   return fetch(
-    corsServerPath + "headers=" + encodeURIComponent(dict.toSource())
+    corsServerPath + "headers=" + encodeURIComponent(JSON.stringify(dict))
   ).then(
     function(res) {
       is(res.status, 200, "expected correct referrer header to be sent");