Joe in my Firefox (14.0.1 under Ubuntu 12.04) it's on Edit/Preferences/Privacy then click on clear your recent history, open up the Details list (it it's not open already), then tick the Cache item (and untick any of the others that you might not want to clear: Cookies etc), set the Time range drop down to your desired value (Everything if you want all) and press the Clear Now button!.

It's easier than it looks written out (not that I've ever bothered!).

Mick