popups.css: Fix various popup menus and installing extensions

This fixes all that has been described in https://linmob.net/mcf06-some-progress-and-a-call-for-feedback/, and additionally fixes #58 (closed), making installing extensions in portrait possible again ESR 115 and 124. Line 54 fixes the overflowMenu, which without this obscured the urlbar (entering customization mode is still possible by long pressing the unified extensions' menu icon).

This change positively affects and should fix the following issues: #40 (closed), #3 (closed)

Additionally, the related issue #54 (closed) should be closed - there's just no more way in current Firefox to move extensions to the overflow menu, as the possible solution has since been removed from Firefox about:config.