I am aware of only one bug report related to 64 bit linux Firefox
3.5.7 and Firebug 1.5.0: an immediate crash. This bug was found this
fall, reported to Mozilla, and even though they don't support 64 bit
builds, this crash was fixed in December.
You can obtain a version of 64 bit Firefox that does not crash on this
bug in two ways: either install Firefox 3.6 or a nightly build of
Firefox 3.5.8.
Firefox > Tools > Addons will only support the latest Firebug version,
currently 1.5.0. If you want to complain about that, you need to
contact addons.mozilla.org. I doubt you will have success, since
mozilla does not support 64 bit builds.
getfirebug.com has all releases of firebug, http://getfirebug.com/releases.
I can't read that page myself, so you can go directly to the firebug
directory with
http://getfirebug.com/releases/firebug.
jjb