Alice wrote:(For the record)
Firefox 3 has a delayed shutdown that causes the issue if you try to restart it to soon after it was last closed. Related bug reports:
Bug 399108 – Need an I'm-not-dead-yet message when minefield takes a while to close
Bug 407981 – Delayed shutdown makes it impossible to start Firefox immediately after exiting
For other causes and solutions see:
http://kb.mozillazine.org/Firefox_hangs#Hang_at_exit
Thanks for providing links to the bug reports and the KB article. I hope this issue will be addressed soon as I'm really thinking about downgrading back to FF2 which has worked well for me during the past, or looking for other alternatives. Needless to say that none of the solutions described in the knowledge base article actually worked for me.