Thanks. This post saved me a lot of debugging and hair.
I'm on ff5, on ubuntu 10.04. fb 1.81
I did updated my whole system via apt-get a few hours ago, and then
restarted my computer. After the restart, fb was no longer accessible.
I did a little googling and found this post. I disabled firephp,
restarted ff and fb was immediately available again.
I'm not sure what apt changed.