One more diagnostic:

when I "quit" (using the "file" menu) and restart firefox, I get the 
error message

"Firefox is already running, but is not responding. To open a new 
window, you must first close the existing Firefox process, or restart 
your system."

This seems to be an indication that lock files are not handled very well 
by firefox, and since both thunderbird and firefox are descendent of 
mozilla, there is something related going on.

At the very least, the fact that firefox lock files needs to be deleted 
manually is something that I think should be fixed for general usership.