I am running firefox 2 and thunderbird 2, and the only problems i have
had are

1) sometimes x runs out of some resource and I get 'bad alloc' or
   something

2) the linux flash plugin (via nspluginwrapper) is buggy (or it's the
   linux emul or the wrapper, but my guess is flash itself) and the
   whole program hangs.  I then "pkill npviewer.bin" and all is well
   again.