1. Open Firefox, Type "about:config" without quotes into the address bar and hit enter/click Go.

2. Right-click anywhere, select New, then Integer. In the dialog prompt that appears, type:

browser.cache.memory.capacity

3. Click OK. Another dialog prompt will appear. This is where you decide how much memory to allocate to Firefox. This depends on how much RAM your computer has, but generally you don't want to allocate too little (under 8MB), but if you allocate too much, you might as well not do this. A good recommended setting is 16MB. If you want 16MB, enter this value into the dialog prompt:

16384

(Why 16384 instead of 16000?
Because computers use base-12 counting. Thus 16 megabytes = 16384 bytes. Likewise, if you want to double that and allocate 32MB, you'd enter 32768.)

4. Click OK to close the dialog box, then close all instances of Firefox and restart. If your Firefox still uses the same amount of memory, give it a few minutes and it should slowly clear up. If that fails, try a system reboot.