If you have plenty of RAM and do no want Firefox to save temporary files to disk (for example because you have SSD and want to limit number of disk writes) you can move Firefox cache to RAM.
There is no way how to do it in gui, so you need to use favorite about:config. Search for parameter browser.cache.disk.enable and set that to false. Now disk cache is disabled. Then search for browser.cache.memory.enable and set that as true – it will enable cache in RAM (it is set to true already sometimes).
If you want to set how much RAM you will dedicate to cache, search for browser.cache.memory.capacity and as value set desired cache size in kilobytes.
Moving cache to RAM is good idea only if you have quick internet connection as cache is wiped out on every restart and all pictures, scripts etc. are downloaded again after restart.