I suggest you carefully repeat the installation in new profile
procedure. My guess is that you accidently re-ran a old profile.
Create the new profile and use Firefox > Tools >Addons to verify that
you have no addons. Then install Firebug 1.4b3 from
http://getfirebug.com/releases.
Don't install anything else. Restart and Firebug will work. (Many
thousands of people have succeeded).
It is funny to see how closely IE8 copies Firebug ;-)
jjb