diff --git a/platform/chromium/vapi-background.js b/platform/chromium/vapi-background.js
index 1736999..40d75c3 100644
--- a/platform/chromium/vapi-background.js
+++ b/platform/chromium/vapi-background.js
@@ -39,7 +39,7 @@ var manifest = chrome.runtime.getManifest();
 
 vAPI.chrome = true;
 
-vAPI.webextFlavor = '';
+vAPI.webextFlavor = undefined;
 if (
     self.browser instanceof Object &&
     typeof self.browser.runtime.getBrowserInfo === 'function'
@@ -47,6 +47,8 @@ if (
     self.browser.runtime.getBrowserInfo().then(function(info) {
         vAPI.webextFlavor = info.vendor + '-' + info.name + '-' + info.version;
     });
+} else {
+    vAPI.webextFlavor = '';
 }
 
 var noopFunc = function(){};
@@ -776,7 +778,11 @@ vAPI.cloud = (function() {
     //  actual data, but all of this is provided for free by browser vendors,
     //  so we need to accept and deal with these limitations.
     var initialize = function() {
-        var ratio = vAPI.webextFlavor.startsWith('Mozilla-Firefox-') ? 0.6 : 0.75;
+        var ratio =
+            vAPI.webextFlavor === undefined ||
+            vAPI.webextFlavor.startsWith('Mozilla-Firefox-') ?
+                0.6 :
+                0.75;
         maxChunkSize = Math.floor(maxChunkSize * ratio);
         initialize = function(){};
     };
diff --git a/src/js/traffic.js b/src/js/traffic.js
index 23b027a..3c2b6ee 100644
--- a/src/js/traffic.js
+++ b/src/js/traffic.js
@@ -1,7 +1,7 @@
 /*******************************************************************************
 
     uMatrix - a Chromium browser extension to black/white list requests.
-    Copyright (C) 2014-2017 Raymond Hill
+    Copyright (C) 2014-2018 Raymond Hill
 
     This program is free software: you can redistribute it and/or modify
     it under the terms of the GNU General Public License as published by
@@ -314,18 +314,16 @@ var onHeadersReceived = function(details) {
         csp.push(µm.cspNoInlineStyle);
     }
 
-    // TODO: Firefox will eventually support `worker-src`:
-    // https://bugzilla.mozilla.org/show_bug.cgi?id=1231788
-    if ( µm.cspNoWorker === undefined ) {
-        µm.cspNoWorker = vAPI.webextFlavor.startsWith('Mozilla-') ?
-            "child-src 'none'; frame-src data: blob: *; report-uri about:blank" :
-            "worker-src 'none'; report-uri about:blank" ;
+    // https://bugzilla.mozilla.org/show_bug.cgi?id=1302667
+    var cspNoWorker = µm.cspNoWorker;
+    if ( cspNoWorker === undefined ) {
+        cspNoWorker = cspNoWorkerInit();
     }
 
     if ( µm.tMatrix.evaluateSwitchZ('no-workers', rootHostname) ) {
-        csp.push(µm.cspNoWorker);
+        csp.push(cspNoWorker);
     } else if ( µm.rawSettings.disableCSPReportInjection === false ) {
-        cspReport.push(µm.cspNoWorker);
+        cspReport.push(cspNoWorker);
     }
 
     var headers = details.responseHeaders,
@@ -365,6 +363,18 @@ var onHeadersReceived = function(details) {
 
 /******************************************************************************/
 
+var cspNoWorkerInit = function() {
+    if ( vAPI.webextFlavor === undefined ) {
+        return "child-src 'none'; frame-src data: blob: *; report-uri about:blank";
+    }
+    µMatrix.cspNoWorker = /^Mozilla-Firefox-5[67]/.test(vAPI.webextFlavor) ?
+        "child-src 'none'; frame-src data: blob: *; report-uri about:blank" :
+        "worker-src 'none'; report-uri about:blank" ;
+    return µMatrix.cspNoWorker;
+};
+
+/******************************************************************************/
+
 // Caller must ensure headerName is normalized to lower case.
 
 var headerIndexFromName = function(headerName, headers) {