Use RAM-only cache
Firefox can be configured to use only RAM as cache storage. Configuration files, bookmarks, extensions etc. will be written to harddisk/SSD as usual. For this
open about:config in the address bar
set browser.cache.disk.enable to "false" (double click the line)
set browser.cache.memory.enable to "true" (double click the line)
set browser.cache.memory.max_entry_size to the ammount of KB you'd like to spare, to -1 for automatic cache size selection
Main disadvantages of this method are that your tabs won't survive a browser crash, and that you need to configure the settings each user individually. On the other hand on a personal system it probably is the easiest method to implement.
Assuming that there is memory to spare, caching all, or part of Firefox’s profile to RAM using tmpfs offers significant advantages. Even though opting for the partial route is an improvement by itself…