diff --git a/modules/libpref/init/all.js b/modules/libpref/init/all.js
index 90dd0925aa..3b46c33b27 100644
--- a/modules/libpref/init/all.js
+++ b/modules/libpref/init/all.js
@@ -4564,6 +4564,8 @@ pref("full-screen-api.enabled", false);
 pref("full-screen-api.unprefix.enabled", true);
 pref("full-screen-api.allow-trusted-requests-only", true);
 pref("full-screen-api.pointer-lock.enabled", true);
+// whether to prevent the top level widget from going fullscreen
+pref("full-screen-api.ignore-widgets", false);
 
 // transition duration of fade-to-black and fade-from-black, unit: ms
 pref("full-screen-api.transition-duration.enter", "0 0");