Reduce the memory usage in Firefox

Besides from disabling some extensions there are a couple of settings you can tweak in Firefox that can limit the amount of memory allocated for caching.

1. Open Firefox and type in about:config in the address bar.

2. Type browser.cache.memory.enable in the Filter Bar and make sure it is enabled. If not, just double click it to set it to enable.

3. Right-click an area of empty space in the browser and choose New > Integer. Type in browser.cache.memory.capacity and click OK.

The integer value you enter depends on the amount of RAM you have.
If you have:
256 MB of RAM, enter “4096”
512 MB of RAM, enter “8192”
1 GB of RAM, enter “16384”
and so forth… Just double the number as your RAM doubles.

4. Right-click anywhere on the window and click New > Bootlean and enter:
config.trim_on_minimize

And set it to true. When you minimize an application in Windows Vista, the memory usage is supposed to go down but Firefox doesn’t do this. By setting this to true, Firefox will begin to use less memory when minimized.

5. Restart Firefox for these changes to apply.

Read more : http://vistarewired.com/2007/04/18/how-to-really-reduce-the-memory-usage-in-firefox/