diff --git a/platform/firefox/vapi-background.js b/platform/firefox/vapi-background.js
index f062a50fd..5e8924e72 100644
--- a/platform/firefox/vapi-background.js
+++ b/platform/firefox/vapi-background.js
@@ -122,6 +122,10 @@ vAPI.browserSettings = {
             return;
         }
         var value = this.originalValues[key];
+        // https://github.com/gorhill/uBlock/issues/292#issuecomment-109621979
+        // Forget the value immediately, it may change outside of
+        // uBlock control.
+        delete this.originalValues[key];
         // Original value was a default one
         if ( value === undefined ) {
             try {