I just wanted to say I have the same problem with Firebug after an auto-update (I'm pretty sure only Firebug was updated, as I remember having Firefox 15 previously. However, I may have had 15.0 before, and it could have just updated to 15.0.1, I'm not entirely sure). I'm using Firefox 15.0.1 Canonical (on Ubuntu 12.04, 64-bit).
Firebug doesn't show up when I start Firefox, and I have to disable/enable it to be able to use F12/right click menu/Navigation bar icon. The first time I disabled & re-enabled I had to get the Nav bar icon from the Customize menu, and now it shows up when I disable/enable.
I also created a fresh profile to test it out, with only Firebug installed, and it does the same thing. If there's anything else I could do to help I'd be glad to.