I see both your points (browser vendors and extension distribution issues), and I think Mozilla has shown great respect for their users. However, Mozilla's primary revenue source is Google.

Maybe taking a page out of the enterprise play book and using a proxy, like Squid, would make sense. From reading the Squid manual, it seems like it could play a role as it is quite extensible and sophisticated. Making it easy to setup and customize would be pretty difficult from what I can tell, unfortunately.