Firefox won't erase the browsing history while you were on Aol with the old password. However, the hacker probably didn't get in with using your firefox saved passwords. There's lots of better ways to get in. But if you don't have any history you want to keep, it would probably be a good idea just in case to delete your history. You can do this by following these steps.

At the top of the Firefox window, click the Firefox button, go over to the History menu and select Clear Recent History.... ... Select how much history you want to clear: ... Finally, click the Clear Now button and the window will close and the items you've selected will be cleared.

Jon

Firefox browser history has nothing to do with security, it simply stores a list of pages visited, not the password used to log in or anything. If Firefox is saving your password, then you'll want to make sure that you update it with the new password. Password manager - Remember, delete and change saved passwords in Firefox

I'd also suggest yo scan your computer for malware. Keyloggers installed on your computer are a very possible way that your password was stolen. Troubleshoot Firefox issues caused by malware