Firefox is really one of the finest browsers and it is best when it comes to customisation. However one negative thing is that its little more bulkier(than Google Chrome) and slow. You might be surprised to know much much RAM your Firefox is consuming. Here is an app (or add on), Memory Restart that lets you know the memory usage by the browser and more importantly gives you a quick way to restart if memory usage is too high and hence prevents it from being slow. Once you install this add on you would be able to see the actual number(of memory usage) on top right. Once it get too high and exceed the limit(decided by you) the number turns out to be red in color.Just click on the number and reloading of browser takes place and the memory occupied will decrease by large amount.