I haven't noticed any issue with Firefox 3 on Windows and I used it solidly all day Monday due to "
issues" with my Linux machine. Since then I upgraded my LinuxMint from V4 to V5, Firefox was upgraded at the same time and has been crashing every few hours.
I have a tentative guess of the cause, one that has been an issue before - Adobe Flash. I'm trying some experiments with blocking flash to see if it becomes stable again. The Windows machine does not have flash installed and is stable whereas the Linux machine reverses both of those.
I'll let you know what (if anything) I find that works for me.