For about a year now, I have had my "/.mozilla mounted as a tmpfs. I don't have a SSD, but I have 2Gb of RAM, and Firefox has never run out of memory for me.
It's screaming fast. I originally started doing this when I had my $HOME mounted over SSHFS, and Firefox would single-handedly saturate my pipe, and took forever to do anything. Its disk IO is (was) obscene.
This also has the benefit (if you want to see it that way) that my history does not get so filled with garbage, since every reboot the profile is reset. I have a line in my .Xclients which copies a template .mozilla into place, so that I start off with Noscript, Adblock, Tor, etc, all enabled, and my history is seeded with LWN and other sites I frequent.