I have had a lot of problems with the debian firefox package. I have had better luck with downloading the nightly 1.5 builds from http://ftp.mozilla.org/pub/mozilla.org/firefox/ . All I did was copy, past and overwrite the files in the /usr/lib/firefox dir. This also fixes the problems many have had with foxytunes not being able to find any player.

Hum to be more precise on my problem:
It crashes if you have Xorg composite extension (transparency) + Flash Plugin

So I removed Flash.
rm ~/.mozilla/plugins/*flash*

Rock stable again :)

I don't think thatI ever reported back, but my issues turned out to be Java-related. At first I dealt with it, but eventually I moved to Kubuntu 6.06 and I've not had issues as of late.