In defense of Firefox

> I'm using Fedora 9 at home and have yet to encounter this behavior. What does it take to
trigger it?

The issue is that Firefox will perform large numbers of fsync system calls on files in
~/.mozilla, and fsync is implemented as a global sync on journaled filesystems.

One way to reproduce the slowdown is to put ~/.mozilla on an ext3 filesystem which has heavy
unrelated write traffic.  Another is to put it on a busy NFS filesystem (having home on NFS is
rather common in academic settings).