Mozilla and Apple forums have few threads complaining of such but there was not a clear answer in any of those, so the problem persisted for me until I found this "Snow Leopard Crashes and Problems" and problem is with the Fonts in Snow Leopard. After the upgrade (esp. if you have extra fonts installed), few of them are corrupted.
So I followed the simple step of reseting the font caches first (as mentioned on the page). Type in terminal "atsutil databases -removeUser" and restart your system so OSX can rebuild the fonts. And after the restart - check the Font Book and remove duplicate fonts or validate fonts, if any. In my system, even after the restart, some fonts were not working - so I revalidated them.
And this seems to resolve the problem of firefox crashing due to font issues (not sure of other applications but some of them are caused by fonts)
Am happy Firefox is working smoothly now and hopefully it will continue to do so :)