There was once a time I would have thought this is bad. Not telling a user what's going on can't be good, right?
I think this is true for some users, but it's a minority of them. Most people don't understand what the update is for, or why they should get it. That means that some of them will click "no" when asked if they should update. In the rare event someone has a need to not take an update, they can choose to go down this dangerous path.
The obvious counter argument is "what if my vendor does something evil with their update!" If this is something you're worried about, you need a new vendor. If you can't trust your vendor, what's worse, a system that can be infected by an evildoer, or a system that IS infected by a dishonest vendor?
Automatic updates for security flaws are good, automatic updates for random vendor whims are not. I suspect that much of the fear of automatic updates comes from vendors trying to sneak in other changes. I would say if you don't trust your vendor, and they don't trust you, what's even the point?
I guess the point is the user can find annoying the feature if the browser is updated frequently. Although a product that it's getting more updated is more secure than the one not being updated, for the user it acts like "if it's being updated, it's because it's bad".
I think it would be better to educate the user, because the annoyance it's not Firefox's fault, but from other products (blame Windows Vista here) that make updates a synonymous of bothersome.
I agree with you this can be a good feature, as long as it's optional and the user gets information in case he chooses it not to be automatic.
My main concern about automatic updates is the scenario where some bugs haven't been caught during QA and these are then pushed out to all users. Allowing people the ability to perform controlled updates means testing can be done first.
This certainly applies in a corporate type environment. When you have someone who knows what they're doing and deploys software, you certainly don't want random applications updating themselves automatically.
In a home user sort of world though, I think automatic updates are best. Most people don't understand what's going on, or even what's at stake by not updating.
Given the recent phony attack site messages bearing viruses (boy, they really Do look like the real thing!), the phony update is probably not far behind - given that, I 'd rather have Firefox doing the updates in the background, so I don't make a regrettable "too quick" decision . . .