Firefox memory.

I've found a page with some useful tricks about FF2.

Something that seems to have and effect on RAM consumption is:

browser.cache.disk.capacity: 5000.

even if I don't really understand why. (Option->Advances->Network->Cache)

More relevant seems to be (from tweakguides):

browser.cache.memory.capacity [Integer] *Create - This setting determines whether Firefox uses system RAM to cache itself, improving performance. A value of 0 turns off caching into RAM, which is not recommended as it can greatly slow down browsing. The default value of -1 tells Firefox to automatically determine the size of the cache based on your physical RAM. In Firefox 2.0, systems with 512MB of RAM will result in a 14MB cache, 1GB of RAM results in a 18MB cache, 2GB of RAM gives 24MB of cache, and 4GB of RAM results in 30MB of cache. These values are lower than the defaults in Firefox 1.5. The default of -1 should be fine for all people, but if you notice Firefox increasing its memory usage, you can manually restrict this value to a particular amount by entering a value in KB (e.g. a value of 18432 = 18MB).

You'll have to create this key.

or

browser.sessionhistory.max_total_viewers [Integer] - This option determines how many pages to store in memory to speed up the back and forward buttons in Firefox. The default of -1 automatically determines the amount based on your system RAM. At 512MB of RAM, 5 pages are held in memory, while 1GB or more of RAM holds 8 pages. You can set this value to 0 to hold no pages in RAM (only recommended for troubleshooting memory problems), or increase the value if you often use the back and forward functions for more than 8 pages.