diff --git a/platform/chromium/vapi-cachestorage.js b/platform/chromium/vapi-cachestorage.js
index e95d22e54..78979feb9 100644
--- a/platform/chromium/vapi-cachestorage.js
+++ b/platform/chromium/vapi-cachestorage.js
@@ -1,7 +1,7 @@
 /*******************************************************************************
 
     uBlock Origin - a browser extension to block requests.
-    Copyright (C) 2016-2017 The uBlock Origin authors
+    Copyright (C) 2016-2018 The uBlock Origin authors
 
     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
@@ -39,6 +39,14 @@
 // indexedDB.
 
 vAPI.cacheStorage = (function() {
+
+    // Firefox-specific: we use indexedDB because chrome.storage.local() has
+    // poor performance in Firefox. See:
+    // https://bugzilla.mozilla.org/show_bug.cgi?id=1371255
+    if ( vAPI.webextFlavor.soup.has('firefox') === false ) {
+        return vAPI.cacheStorage;
+    }
+
     const STORAGE_NAME = 'uBlock0CacheStorage';
     var db;
     var pending = [];
diff --git a/platform/chromium/vapi-usercss.js b/platform/chromium/vapi-usercss.js
new file mode 100644
index 000000000..25e6e10cc
--- /dev/null
+++ b/platform/chromium/vapi-usercss.js
@@ -0,0 +1,32 @@
+/*******************************************************************************
+
+    uBlock Origin - a browser extension to block requests.
+    Copyright (C) 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
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see {http://www.gnu.org/licenses/}.
+
+    Home: https://github.com/gorhill/uBlock
+*/
+
+'use strict';
+
+// This file can be replaced by platform-specific code. If a platform is
+// known to NOT support user stylsheets, vAPI.supportsUserStylesheets can be
+// set to `false`.
+
+// 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);
diff --git a/platform/chromium/vapi-usercss.pseudo.js b/platform/chromium/vapi-usercss.pseudo.js
index 142bab49e..d23336ef8 100644
--- a/platform/chromium/vapi-usercss.pseudo.js
+++ b/platform/chromium/vapi-usercss.pseudo.js
@@ -21,20 +21,10 @@
 
 'use strict';
 
-// For content pages
+// Packaging this file is optional: it is not necessary to package it if the
+// platform is known to support user stylesheets.
 
-// Abort execution if our global vAPI object does not exist.
-//   https://github.com/chrisaljoudi/uBlock/issues/456
-//   https://github.com/gorhill/uBlock/issues/2029
-
-// https://github.com/gorhill/uBlock/issues/3588
-//   Chromium 66+ supports user stylesheets. Testing support against a regex is
-//   (unfortunately) necessary from content scripts.
-
-if (
-    typeof vAPI === 'object' &&
-    /\bEdge\/\d+|\bChrom(?:e|ium)\/(?:[45][0-9]|6[0-5])/.test(navigator.userAgent)
-) {
+if ( typeof vAPI === 'object' && vAPI.userStylesheet === undefined ) {
 // >>>>>>>> start of HUGE-IF-BLOCK
 
 /******************************************************************************/
diff --git a/platform/chromium/vapi-usercss.real.js b/platform/chromium/vapi-usercss.real.js
index 493a4b246..9703fd9dd 100644
--- a/platform/chromium/vapi-usercss.real.js
+++ b/platform/chromium/vapi-usercss.real.js
@@ -21,12 +21,10 @@
 
 'use strict';
 
-// For content pages
+// Packaging this file is optional: it is not necessary to package it if the
+// platform is known to not support user stylesheets.
 
-if (
-    typeof vAPI === 'object' &&
-    vAPI.userStylesheet === undefined
-) {
+if ( typeof vAPI === 'object' && vAPI.supportsUserStylesheets ) {
 // >>>>>>>> start of HUGE-IF-BLOCK
 
 /******************************************************************************/
diff --git a/platform/debian/vapi-usercss.js b/platform/debian/vapi-usercss.js
new file mode 100644
index 000000000..3013e50eb
--- /dev/null
+++ b/platform/debian/vapi-usercss.js
@@ -0,0 +1,32 @@
+/*******************************************************************************
+
+    uBlock Origin - a browser extension to block requests.
+    Copyright (C) 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
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see {http://www.gnu.org/licenses/}.
+
+    Home: https://github.com/gorhill/uBlock
+*/
+
+'use strict';
+
+// This file can be replaced by platform-specific code. If a platform is
+// known to NOT support user stylsheets, vAPI.supportsUserStylesheets can be
+// set to `false`.
+
+// 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);
diff --git a/platform/webext/vapi-usercss.js b/platform/webext/vapi-usercss.js
new file mode 100644
index 000000000..0daf2f7d2
--- /dev/null
+++ b/platform/webext/vapi-usercss.js
@@ -0,0 +1,26 @@
+/*******************************************************************************
+
+    uBlock Origin - a browser extension to block requests.
+    Copyright (C) 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
+    the Free Software Foundation, either version 3 of the License, or
+    (at your option) any later version.
+
+    This program is distributed in the hope that it will be useful,
+    but WITHOUT ANY WARRANTY; without even the implied warranty of
+    MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+    GNU General Public License for more details.
+
+    You should have received a copy of the GNU General Public License
+    along with this program.  If not, see {http://www.gnu.org/licenses/}.
+
+    Home: https://github.com/gorhill/uBlock
+*/
+
+'use strict';
+
+// User stylesheets are always supported with Firefox/webext .
+
+vAPI.supportsUserStylesheets = true;
diff --git a/tools/make-chromium.sh b/tools/make-chromium.sh
index dbec5535f..4bc039165 100755
--- a/tools/make-chromium.sh
+++ b/tools/make-chromium.sh
@@ -24,14 +24,17 @@ cp platform/chromium/*.json $DES/
 cp LICENSE.txt              $DES/
 
 echo "*** uBlock0.chromium: concatenating content scripts"
-cat $DES/js/vapi-usercss.pseudo.js > /tmp/contentscript.js
+cat $DES/js/vapi-usercss.js > /tmp/contentscript.js
 echo >> /tmp/contentscript.js
 grep -v "^'use strict';$" $DES/js/vapi-usercss.real.js >> /tmp/contentscript.js
 echo >> /tmp/contentscript.js
+grep -v "^'use strict';$" $DES/js/vapi-usercss.pseudo.js >> /tmp/contentscript.js
+echo >> /tmp/contentscript.js
 grep -v "^'use strict';$" $DES/js/contentscript.js >> /tmp/contentscript.js
 mv /tmp/contentscript.js $DES/js/contentscript.js
-rm $DES/js/vapi-usercss.pseudo.js
+rm $DES/js/vapi-usercss.js
 rm $DES/js/vapi-usercss.real.js
+rm $DES/js/vapi-usercss.pseudo.js
 
 # Chrome store-specific
 cp -R $DES/_locales/nb $DES/_locales/no
diff --git a/tools/make-webext.sh b/tools/make-webext.sh
index e7207b20d..4a1648673 100755
--- a/tools/make-webext.sh
+++ b/tools/make-webext.sh
@@ -26,16 +26,19 @@ cp LICENSE.txt                          $DES/
 
 cp platform/webext/manifest.json        $DES/
 cp platform/webext/polyfill.js          $DES/js/
+cp platform/webext/vapi-usercss.js      $DES/js/
 cp platform/webext/vapi-webrequest.js   $DES/js/
-cp platform/webext/vapi-cachestorage.js $DES/js/
 
 echo "*** uBlock0.webext: 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.pseudo.js
+rm $DES/js/vapi-usercss.js
 rm $DES/js/vapi-usercss.real.js
+rm $DES/js/vapi-usercss.pseudo.js
 
 # Webext-specific
 rm $DES/img/icon_128.png