I'm using the latest stable Firefox under Ubuntu 13.04 x64. I disabled menu bar, because using the menu button is more comfortable for me. Until an update (around version 21 or 22) the button just disappeared. I checked the tool bar settings but those are correct, so nothing has been changed. I purged Firefox and removed the cache, but it is still the same. BUT I noticed an exception: if I run Firefox with sudo, the menu button appears. I'm talking about this little thing what I miss.
How can I get it back permanently?