Mobile/MemoryReduction

Reduce session history entries

user_pref("browser.sessionhistory.max_entries", 0);

Turn off bfcache

user_pref("browser.sessionhistory.max_total_viewers", 0);

Disable X image caching on Unix/Linux

An environmental variable. Set it like this:

MOZ_DISABLE_IMAGE_OPTIMIZE=1 ./firefox

It stops images from being cached on the X server on Unix/Linux. Generally hurts performance, but gives you a more predictable pixmap usage on the server.

Reduce memory cache

user_pref("browser.cache.memory.enable", false);

This turns off the cache - stuart says this should be set to some minimal number otherwise you'll do a lot more decoding images over and over again. Need to do some investigation.

Reduce disk cache

user_pref("browser.cache.disk.capacity", value);

Untested. Setting this too low will break ajaxy sites.