"This can be quite annoying. For example, AdBlockPlus is extremely popular as an extension for blocking pop-up ads and banners, but (sometimes unintentionally) there are quite a few Firefox extensions that disrupt it, drawing regular complaints from people trying new extensions, or upgrading existing ones. Mozilla is now proposing a set of policies to police this problem, prompted by a recent catfight between extension developers. The proposed policy changes raise questions about just how much freedom extension developers should enjoy."
