proners wrote"Some addons may cause memory leaks"
That, plus compatibility issues among addons (one addon having an effect on another, etc...) is what I would call misbehaving addons.
Also, other weird stuff can happen, even after you uninstall addons. For example my menubar used to show/hide in the following fashion.. the menubars visibility on firefox startup would be the opposite of what it was when I quit the application. For example if I closed firefox and the menubar was hidden, it would be shown when I start firefox again, and the opposite being true.
Fixed it by editing a chrome css file... but I wonder where that came from, I think it has something to do with addons.