For some reason, Mozilla is saving the cache in the wrong place, and I'm unable to find out how to change it to the right place.

It's storing the cache in /home/username/.cache right now, instead of using /tmp which is meant for this. I'm new to FireFox and I've only been using it for a week, for just a few websites, and I already have a 1.5GB folder in my .cache folder, that should never have been there in the first place.