Here's a useful addon. You may think emptying the cache is of little importance but it can be very helpful.
- Cleans out cached images
- Free disk space
- May help fix browser issues
- Help protect privacy
Donload the addon
HERE Just tick the latest version ie. 1.3
Once installed you need to restart Firefox, then right click on the toolbar select customise find the "Empty Cache" icon and drag it onto the toolbar and your done.