diff --git a/platform/chromium/vapi-usercss.js b/platform/chromium/vapi-usercss.js
index 25e6e10cc..1a2d73a41 100644
--- a/platform/chromium/vapi-usercss.js
+++ b/platform/chromium/vapi-usercss.js
@@ -28,5 +28,7 @@
 // Chromium 66 and above supports user stylesheets:
 // https://github.com/gorhill/uBlock/issues/3588
 
-vAPI.supportsUserStylesheets =
-    /\bChrom(?:e|ium)\/(?:6[6789]|[789]|1\d\d)/.test(navigator.userAgent);
+if ( typeof vAPI === 'object' ) {
+    vAPI.supportsUserStylesheets =
+        /\bChrom(?:e|ium)\/(?:6[6789]|[789]|1\d\d)/.test(navigator.userAgent);
+}
diff --git a/platform/debian/vapi-usercss.js b/platform/debian/vapi-usercss.js
index 3013e50eb..0f3b4c283 100644
--- a/platform/debian/vapi-usercss.js
+++ b/platform/debian/vapi-usercss.js
@@ -28,5 +28,7 @@
 // Chromium 66 and above supports user stylesheets:
 // https://github.com/gorhill/uBlock/issues/3588
 
-vAPI.supportsUserStylesheets =
-    /\bChrom(?:e|ium)\/(?:6[6789]|[789]|1\d\d)|\bFirefox\/\d/.test(navigator.userAgent);
+if ( typeof vAPI === 'object' ) {
+    vAPI.supportsUserStylesheets =
+        /\bChrom(?:e|ium)\/(?:6[6789]|[789]|1\d\d)|\bFirefox\/\d/.test(navigator.userAgent);
+}
diff --git a/platform/webext/vapi-usercss.js b/platform/webext/vapi-usercss.js
index 0daf2f7d2..b3a3ce91e 100644
--- a/platform/webext/vapi-usercss.js
+++ b/platform/webext/vapi-usercss.js
@@ -23,4 +23,6 @@
 
 // User stylesheets are always supported with Firefox/webext .
 
-vAPI.supportsUserStylesheets = true;
+if ( typeof vAPI === 'object' ) {
+    vAPI.supportsUserStylesheets = true;
+}
diff --git a/tools/make-firefox.sh b/tools/make-firefox.sh
index e0b68872b..b6f8c8398 100755
--- a/tools/make-firefox.sh
+++ b/tools/make-firefox.sh
@@ -22,6 +22,7 @@ cp    platform/firefox/css/*            $DES/css/
 cp    platform/firefox/polyfill.js      $DES/js/
 cp    platform/firefox/vapi-*.js        $DES/js/
 cp    platform/chromium/vapi-usercss.real.js $DES/js/
+cp    platform/webext/vapi-usercss.js   $DES/js/
 cp    platform/firefox/bootstrap.js     $DES/
 cp    platform/firefox/processScript.js $DES/
 cp    platform/firefox/frame*.js        $DES/
@@ -32,10 +33,13 @@ cp    platform/firefox/*.xul            $DES/
 cp    LICENSE.txt                       $DES/
 
 echo "*** uBlock0.firefox: concatenating content scripts"
-cat $DES/js/vapi-usercss.real.js > /tmp/contentscript.js
+cat $DES/js/vapi-usercss.js > /tmp/contentscript.js
+echo >> /tmp/contentscript.js
+grep -v "^'use strict';$" $DES/js/vapi-usercss.real.js >> /tmp/contentscript.js
 echo >> /tmp/contentscript.js
 grep -v "^'use strict';$" $DES/js/contentscript.js >> /tmp/contentscript.js
 mv /tmp/contentscript.js $DES/js/contentscript.js
+rm $DES/js/vapi-usercss.js
 rm $DES/js/vapi-usercss.real.js
 
 echo "*** uBlock0.firefox: Generating meta..."