You've got to figure some extensions'll be made by the public. From the specs, it sounds like google will be building extensions. Imo this is a good change, I've always thought firefox should have a team making & maintaining extensions.
"... I've always thought firefox should have a team making & maintaining extensions ..."
Probably not a good idea for most (aside from Google who have the resources). Most projects concentrate on the core api, documentation and leave the the add-ons for users. I'm specifically thinking of Firefox, JQuery etc.
Google has been bankrolling mozilla for the past couple years now. It's probably in mozilla's interest anyway since extensions provide some good lock-in.
Does that include a mechanism to block textual adverts as well as image based adverts?