This next hack deals with the memory Firefox uses as cache. In the default setting, Firefox will adjust the cache size to fit however many pages you have open. While this is useful, if you tend to only have a few pages open at a time, you can manually reduce the setting to a specified amount at the cost of reducing performance when Firefox runs out of cache. To implement this hack, follow these steps.
1. With Firefox up and running, type about:config and hit enter.
2. Right click on the new page and select New -> Integer.
3. In the pop-up window, type in “browser.cache.memory.capacity”. In the following pop-up window, specify how many Kb of ram you want to dedicate to the cache, I selected 32768(32Mb). I suggest a number between 16Mb and 64Mb, anything lower and performance will suffer, anything higher is excessive. Here you should experiment, or don’t bother if it worries you.
4. Restart your browser
I don’t know how much this helps or hurts performance, so far with 32Mb enabled I have yet to see a major difference, memory use dropped from 42Mb to 39Mb with the same pages open, but I don’t think this hack is what did that.But I'm not sure if its useful or not