Firefox fixed a number of memory leaks in version 18.104.22.168, so make sure you have the latest stable version. Now let's optimize Firefox to use less memory and to work faster.
Type about:config in the address bar.
* create a new boolean entry config.trim_on_minimize and set it to true
* set browser.cache.memory.capacity to a fixed size (the default values are: for 256 MB RAM - 14336, 512 MB RAM - 22528, 1 GB RAM - 32768). A good value to try is 16384.
* set browser.sessionhistory.max_total_viewers to 0
* change image.animation_mode to none
Go to Tools/Options.
* set the disk cache memory to a low value (less than 50 MB)
* turn off Java
* try to avoid these extensions: Adblock, Fasterfox, IE Tab, Session Saver. Use Adblock Plus instead of Adblock.
* don't use more than 5 extensions
Using these tips, I could make Firefox use 50 MB of RAM on average, from more than 100 MB of RAM before.
If you want to fully optimize your memory, you should try FreeRAM XP Pro, a free program that recovers memory leaks.