...and this is what bothers me a little.
Many of those add-ons start as GreaseMonkey scripts. Anyone can post a script there, and it's up to you, if you want to install it, or not.
Then, if the script gets Mozilla approval, it goes on official Mozilla web site.
My question is, why do they approve some add-ons, which are later listed on another page, as problematic add-ons:
http://kb.mozillazine.org/Problematic_extensions