Hi community,
maybe my experience sounds familiar or maybe it helps one of you to save some time of debugging:
Today, my firebug UI was empty (all Grey); firebug 2.0.8 on firefox 35.0.1.
Long story short:
I found that the sub menu Tools > Web Developer > Firebug was shown twice.
I assumed an installation error; removing and re-installing firebug fixed the problem.
Unfortunately, I cannot tell steps to reproduce (I haven't used firebug for a while; automatic updates are on) and cannot write a bug report for that ...
Regards,
Martin