Why has Firefox rocketed in popularity when Mozilla has been around forever? Partly because they stripped out the mail/news reader and all of the other bloat that was unnecessary for a good web browser. ~4 MB download for an excellent browser. That's all I want and need.
The direction of Firefox specifically should proceed further down that road. Fix the bugs, make sure rendering is perfect according to web standards, and focus on the browsing experience. Continue to refine security and privacy features.
Plug-ins are fine; they leave the choice of including them to the user. But for Mozilla, just leave the browser lightweight and work on the way it does its job.