I had the same issue on x64 fedora 11, but found a workaround (although a bit annoying one). Delete the .mozilla directory and start firefox, install firebug, t

I had the same issue on x64 fedora 11, but found a workaround (although a bit annoying one). Delete the .mozilla directory and start firefox, install firebug, then it works. I tried messing with files left behind from the previous firebug individually, but couldn't find a specific culprit... Annoying bug, this is just a temporary workaround.