You probably have ~/.mozilla or whatever firebird uses owned by root. This happens if you've run mozilla/firebird for the first time with sudo. This should fix it:
Did you by chance just install some extensions? Ive had this happen to me before after installing them.
I ended up deleting the /usr/lib/MozillaFirebird/chrome (if you unmerge firebird this directory does not get deleted) and reinstalling firebird and everything worked fine.
I am just now experiencing this problem using mozilla-firebird-0.7. It was working fine yesterday under Gnome2, and then unexpectatedly it wouldn't open after closing all my browser windows. This occurred while logged into the same gnome session.
There were no error messages, and everything seems as though the MozillaFirebird processes are running as normal. Comparison of ps -aux when running Firebird as root (and seeing the windows) versus ps -aux as any user (without seeing the windows) were functionally the same.
I created a new account to test if some local environment variables were the cause to the same result. I tested it under different window managers. I re-emerged Firebird with fingers crossed to no avail. So now I have manually removed /usr/lib/MozillaFirebird and am re-emerging again (I thought ccache would speed this up!).
This is strange, like I'm using Window$ or something...
I am now going to remove it again and try deleting my .mozilla and .pheonix dir's as well as /usr/lib/MozillaFirebird... Will touch back with my findings )