The problem is that everyone and their dogs are starting to develop Mozilla addons and force them on the visiting user. Just like it was earlier with IE. They could well become a vector of insecurity and instability for Mozilla browsers. Soon, the inability of browser to support Mozilla addons could become a virtue equal to inability to support ActiveX. Like, in Opera browser.
You know, Adobe had ported their "download manager" to Firefox. When you are going to download Adobe Reader, they offer you to download it through their addon. Earlier this happened only when you used IE. With Opera, fortunately, it does not happed still. Anyway, this is a sadness to know about Firefox.
The really useful addons are very few in count. But how many addons a typical user would install in their Firefox, you could easily imagine. The time is coming.