How to lower the amount of memory that Firefox uses

If your computer starts to slow down after you’ve been using Firefox for a while, you may be able to regain some of your system resources by limiting the amount of memory that Firefox is “allowed” to use for caching, thus improving your system performance. It should be noted that if you have 1GB or more of RAM, you probably don’t need to follow these steps..
#1. In your Firefox address bar, enter the following text: about:config
2. Scroll down to the entry titled browser.cache.memory.enable and make sure the Value is set to true. If it’s not, double-click anywhere in the browser.cache.memory.enable line and the status will change to true
3. Right-click (Apple users ctrl-click) anywhere in the about:config window and select New and then Integer from the pop-up menu
4. Enter browser.cache.memory.capacity in the New integer value pop-up box
5. You’ll need to enter a number in the Enter integer value, but that number depends on how much memory you have (specifically, how much you want to “give” Firefox). If you have 256MB or less RAM, enter 4096 as the value. If you have 512MB of RAM, enter 6144 or 8192. If you have 128MB or less RAM, you may want to consider upgrading - memory is getting very inexpensive.
6. Confirm that the new entry has been created and the integer value is correct
7. 7. If for any reason you wish to restore the default settings, change the browser.cache.memory.capacity integer value to -1 (that’s minus one).

Restart Firefox and see if the change helped. You can use the Task Manager (Windows) or the Activity Monitor utility (Mac) to monitor your Firefox memory usage.

More info on this subject can be found here: http://kb.mozillazine.org/Browser.cache.memory.capacity