diff --git a/platform/firefox/frameModule.js b/platform/firefox/frameModule.js
index 6f2c284c2..1b365e292 100644
--- a/platform/firefox/frameModule.js
+++ b/platform/firefox/frameModule.js
@@ -319,15 +319,31 @@ var contentObserver = {
             }
 
             sandbox.injectScript = function(script) {
-                var svc = Services;
+                let svc = Services;
                 // Sandbox appears void.
                 // I've seen this happens, need to investigate why.
-                if ( svc === undefined ) {
-                    return;
-                }
+                if ( svc === undefined ) { return; }
                 svc.scriptloader.loadSubScript(script, sandbox);
             };
 
+            sandbox.injectCSS = function(sheetURI) {
+                try {
+                    let wu = win.QueryInterface(Ci.nsIInterfaceRequestor)
+                                .getInterface(Ci.nsIDOMWindowUtils);
+                    wu.loadSheetUsingURIString(sheetURI, wu.USER_SHEET);
+                } catch(ex) {
+                }
+            };
+
+            sandbox.removeCSS = function(sheetURI) {
+                try {
+                    let wu = win.QueryInterface(Ci.nsIInterfaceRequestor)
+                                .getInterface(Ci.nsIDOMWindowUtils);
+                    wu.removeSheetUsingURIString(sheetURI, wu.USER_SHEET);
+                } catch (ex) {
+                }
+            };
+
             // The goal is to have content scripts removed from web pages. This
             // helps remove traces of uBlock from memory when disabling/removing
             // the addon.
@@ -337,8 +353,10 @@ var contentObserver = {
             sandbox.outerShutdown = function() {
                 sandbox.removeMessageListener();
                 sandbox.addMessageListener =
+                sandbox.injectCSS =
                 sandbox.injectScript =
                 sandbox.outerShutdown =
+                sandbox.removeCSS =
                 sandbox.removeMessageListener =
                 sandbox.rpc =
                 sandbox.sendAsyncMessage = function(){};
diff --git a/platform/firefox/vapi-client.js b/platform/firefox/vapi-client.js
index 6bff1b684..a7662a804 100644
--- a/platform/firefox/vapi-client.js
+++ b/platform/firefox/vapi-client.js
@@ -437,6 +437,46 @@ vAPI.messaging.start();
 
 /******************************************************************************/
 
+vAPI.userCSS = (function() {
+    if ( !self.injectCSS ) {
+        return;
+    }
+    var injectCSS = self.injectCSS,
+        removeCSS = self.removeCSS,
+        userCSS = '',
+        sheetURI = '';
+    var load = function() {
+        if ( userCSS === '' || sheetURI !== '' ) { return; }
+        sheetURI = 'data:text/css;charset=utf-8,' + encodeURIComponent(userCSS);
+        injectCSS(sheetURI);
+    };
+    var unload = function() {
+        if ( sheetURI === '' ) { return; }
+        removeCSS(sheetURI);
+        sheetURI = '';
+    };
+    var add = function(cssText) {
+        if ( cssText === '' ) { return; }
+        if ( userCSS !== '' ) { userCSS += '\n'; }
+        userCSS += cssText;
+        unload();
+        load();
+    };
+    var toggle = function(state) {
+        if ( userCSS === '' ) { return; }
+        if ( state === undefined ) {
+            state = sheetURI === '';
+        }
+        return state ? load() : unload();
+    };
+    return {
+        add: add,
+        toggle: toggle
+    };
+})();
+
+/******************************************************************************/
+
 // No need to have vAPI client linger around after shutdown if
 // we are not a top window (because element picker can still
 // be injected in top window).
diff --git a/src/js/contentscript.js b/src/js/contentscript.js
index 675fdde18..1b8d32ff2 100644
--- a/src/js/contentscript.js
+++ b/src/js/contentscript.js
@@ -163,7 +163,8 @@ var allExceptions = Object.create(null),
     allSelectors = Object.create(null),
     commitTimer = null,
     stagedNodes = [],
-    matchesProp = vAPI.matchesProp;
+    matchesProp = vAPI.matchesProp,
+    userCSS = vAPI.userCSS;
 
 // Complex selectors, due to their nature may need to be "de-committed". A
 // Set() is used to implement this functionality.
@@ -428,6 +429,9 @@ var domFilterer = {
                 document.head.appendChild(styleTag);
             }
             this.styleTags.push(styleTag);
+            if ( userCSS ) {
+                userCSS.add(styleText);
+            }
         }
 
         // Simple selectors: incremental.
@@ -580,10 +584,16 @@ var domFilterer = {
     },
 
     toggleOff: function() {
+        if ( userCSS ) {
+            userCSS.toggle(false);
+        }
         this.enabled = false;
     },
 
     toggleOn: function() {
+        if ( userCSS ) {
+            userCSS.toggle(true);
+        }
         this.enabled = true;
     },