if you don't have any other form of backup of your hard-disk available then i fear that this data is lost for good.

for future reference: all your important personal data in firefox is saved in the profile folder, so a backup every now & then might help avoiding another loss of data: Back up and restore information in Firefox profiles

If you deleted the history, like amitshree said, it probably won't be able to be recovered, you just might have to look and find the websites again.

You can try to check the cache via the about:cache page to see if that shows web pages that you recognize.