I had a lot of them. Quite a few that did not have any indication of what they were which makes it necessary to jump through some additional hoops.
Go to "Help" and click on "Troubleshooting Information" and you can find the ID of the add-ons you have installed.
This add-on can be used to clear all those preferences left by extensions after uninstallation, all those garbage which some developers do not care on clearing themselves on their add-ons and which later or sooner will make Firefox to load slower...
eCleaner :: Add-ons for Firefox