Looks like the latest version of Firebug (as of today, Jan. 27, 2010) crashes Firefox in 64-bit Ubuntu when you try to open it. This is unfortunate, so to fix it you have to install an older version of Firebug:
1) Uninstall Firebug from the addons menu.
2) Go here: http://getfirebug.com/releases/firebug/1.4/
3) Grab firebug-1.4.5.xpi from the bottom, and install it.