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 as well.
To get started, all you need to do is install a copy of about:addons-memory for your Firefox browser.
Next, just type about:addons-memory in the address bar, no restart needed. The memory used by each add-on will be displayed.
Now you'll be able to access the memory usage for all of your add-ons in a more efficient manner. If you're having issues with Firefox running slowly or consuming a large chunk of memory, it might be time to remove some of the more greedy add-ons. Sometimes there are alternatives that make better uses of memory and can keep your Firefox running smooth.
Have you found any serious memory hogs? Call them out in the comments!