Been fighting this one for a while now. Running Dapper Kubuntu on amd64.
The main place this shows up for me is on Digg.com, when trying to show buried comments. I've been looking for weeks for the cause, and using Konqueror to surf Digg.
Before I used Kubuntu, I've used both SUSE 10.1 and Gentoo on amd64 and had no problems. Doesn't seem to be either Firefox or the libraries.
I think it is amd64 specific based on research I've done. I am using the most recent build in the repos (220.127.116.11) and the problem is still there (it has been there in the two previous versions as well).
I am a developer, but not really familiar with Firefox (mainly a Java and Python guy professionally)...if I could get someone to point me in the right direction, I'd be happy to take a look at the problem and see if I can track it down.