Did you try telling Firefox to clear the browser history or did you only try to do "select all - delete"?
History menu > Clear All history
Choose 'everything' option
OK
I am waited lot more and ff still not alive. Thank you anyway but problem is solved!FF is kinda slow at clearing History. Just select all and Clear, then let it run. It could take the better part of an hour. I had 8ish months take over 10 minutes, it looked like it crashed, but eventually it was done. It could really use a progress bar or something. Software that leaves people wondering if it is still working is annoying, but they exist.
This type of problem could also be indicative of a corrupted FF profile.