diff --git a/platform/firefox/vapi-background.js b/platform/firefox/vapi-background.js
index b622458e8..360cc5aad 100644
--- a/platform/firefox/vapi-background.js
+++ b/platform/firefox/vapi-background.js
@@ -2240,7 +2240,10 @@ vAPI.net.registerListeners = function() {
         var browser = e.target;
 
         // I have seen this happens (at startup time)
-        if ( !browser.currentURI ) {
+        // https://github.com/gorhill/uBlock/issues/948
+        //   On older version of Firefox, `browser.webNavigation` can be null,
+        //   which would cause currentURI to fail.
+        if ( !browser.webNavigation || !browser.currentURI ) {
             return;
         }
 
diff --git a/src/js/cosmetic-filtering.js b/src/js/cosmetic-filtering.js
index aa3b6941e..dc7e3bcd0 100644
--- a/src/js/cosmetic-filtering.js
+++ b/src/js/cosmetic-filtering.js
@@ -738,7 +738,7 @@ FilterContainer.prototype.compileGenericSelector = function(parsed, out) {
         if ( this.isValidSelector(selector) ) {
             out.push(
                 'c\vlg+\v' +
-                 matches[0] + '\v' +
+                matches[0] + '\v' +
                 selector
             );
         }
@@ -773,7 +773,7 @@ FilterContainer.prototype.compileGenericSelector = function(parsed, out) {
     if ( matches && matches.length === 2 ) {
         out.push(
             'c\vlg+\v' +
-             matches[1] + '\v' +
+            matches[1] + '\v' +
             selector
         );
         return;