Try this
Type about
:config in the address bar.
- Create a new Boolean entry config.trim_on_minimize and set it to true (This one works gr8 when ever you reduce the FireFox window. It almost saves 20 MB – 30 MB+)
- 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). This is free 22MB max.
- Set browser.sessionhistory.max_total_viewers to 0. It can reduce around 20 MB of usage
- Change image.animation_mode to none. It has little impact on performance.
Now go to Tools/Options.
- Set the disk cache memory to a low value (less than 50 MB).
- Turn off Java (If you really don’t use it).
- Try to avoid memory hungry extension. If you decide to use some; don’t use more that 5.
- Clear the download history now and then. Don’t let the list to accumulate. This will have impact on the performance.