using 1.5 and i crashed today like a dozen or more times. Had to go to ie for a stable web browsing experience. A reboot, and the problem seems to have gone away.
You may have to remove/redo the profiles folder that is stored in C:\Documents and Settings\"Some User Profile"\Application Data\Mozilla\Firefox
The migrations of different older Firefox profiles can make Firefox work funky (including the pluginreg.dat file). It's not a great thing to do (having to do that between significant revisions), but what can you really do? I'm pretty sure the thing that can usually be usually saved is the bookmarks file (it's just an html file).