firefox: Let fullscreen fill window not the screen

The WM should be in charge of that.

.config/firefox/user.js

@@ -10,6 +10,7 @@ user_pref("browser.shell.checkDefaultBrowser", true);
 user_pref("browser.startup.page", 3);
 user_pref("browser.urlbar.placeholderName", "DuckDuckGo");
 user_pref("datareporting.healthreport.uploadEnabled", false);
+user_pref("full-screen-api.ignore-widgets", true);
 user_pref("network.cookie.lifetimePolicy", 2);
 user_pref("privacy.donottrackheader.enabled", true);
 user_pref("privacy.trackingprotection.enabled", true);