If you are using a non-factory keyboard (e.g. Swype or AI-type), it's possible that Firefox could be having trouble in launching the keyboard and, therefore, browser may crash.
Another possible explanation would be corrupted data. Try to go to System Settings, Applications, Manage Applications, Firefox, Clear data. This way you will remove all Firefox profiles and data and start over again.
I'm using the default LG keyboard (LG Lucid) but I do notice it seems to have trouble with input in general. It switches between the normal single line cursor and an orange square cursor. Then typing gets very choppy, backspace actually adds text.