Using many Firefox add-ons at once can cause the application to dramatically slow down. This is because for each add-on, you're using a piece of your computer's memory. The most obvious solution is to uninstall some of them, but perhaps that's not a good option for you.
As an alternative, try checking out how much memory each add-on is using. Firefox provides information on memory allocation (about:memory), but it isn't in the most user-friendly format. The next best option is to install yet another add-on. However, this particular add-on will report its own memory usage … Read more