It can take me a while to catch on…
I was wondering for the n-th time whether Firefox was a lot more sluggish than it used to be or if it was just my faulty memory when it hit me. I set my account to use FileVault a while back. Encryption is slow. Firefox puts your profile in your home directory. Your profile contains your cache. The cache gets used a lot…
Fortunately you can move the cache to a different location by setting the browser.cache.disk.parent_directory property in the about:config page. I already had a /NonEncrypted directory used as scratch space for stuff I don’t want slowed down by FileVault, so I just made Firefox point at a /NonEncrypted/Cache/Firefox directory.
Firefox is now indeed a lot more responsive than it was before, and it only took me over a year to figure it out. :P