Most modern browsers have a method of clearing their cache and history in one easy move. In Firefox this can be done by clicking the bookmark button at the top right then selecting History clear recent history. After you have done this you will get the selection for how much history you want to clear, usually this will be Everything:
Once the 'Clear Now' button has been pressed it might take sometime as there might be quite alot of data to clear if it has not been done in some time. Be aware this will also clear any save passwords.