The problem isn't caused by a UI-altering extension, but a simple theme: > 2. Location bar is too big. (Only in Firefox 31 and newer.) https://addons.mozilla.

The problem isn't caused by a UI-altering extension, but a simple theme: > 2. Location bar is too big. (Only in Firefox 31 and newer.) https://addons.mozilla.org/en-US/firefox/addon/firefox-2-theme-for-firefox-3x/ https://addons.mozilla.org/en-US/firefox/addon/firefox-2-the... Looking at the version history of the theme, it looks like the author has been heroically trying to shield users from changes. I'd be pretty annoyed if I had written some addon in the distant past and were forced to make fixes because the Firefox devs broke something that should have been stable 20 versions ago. Firefox's main selling point (in my opinion) is its vast library of high-quality addons and themes. They should be doing everything possible to preserve backward-compatibility, not breaking large swaths of their library.