As I've said before one thing I can't stand about firefox is that its history option is to remember history for "at least x" days. This is completely counter intuitive as it would be a lot more logical to remember history "for at most x" days so that the history doesn't get too big. After searching on mozilla forums I found an answer, and it's documented here.
Basically, type in about:config in the URL bar and then type in "browser.history" and you should see an entry that says browser.history_expire_days and modify that to say the amount that you want. This is the "keep history for at most x days" option. Just in case I set my "keep my history for at least" option which is the browser.history_expire_days_min as one because I wasn't sure what would happen if the min option was larger than the "max" option where it would for example keep the history for at least 5 days but at the same time for at most 2 days which is illogical. So in the end for me I set my max as 7 and when I went over to my history it had auto deleted itself to my relief.
This is what I mean