I used to use this when I still had a hard drive, but this does nothing for performance if you're on an SSD and profile writes are so few with browsers that it doesn't significantly affect drive wear. In the end, all this does is make it more likely that something will break.
Wrong. Using inotify-wait (inotify-tools), you see that FF has a bunch of read and write access on every page load (mostly in <profile>/storage). This is with the about:config option to use RAM as cache enabled.