The problem lies in that not all users know anything beyond point and click. For these users, getting to a site that says "You will need the flash plug in to view this site correctly" is a deal breaker. Even more so when all they see is just some inocous little image that doesnt explain to them why it isn't working. (Ala the little jigsaw piece)
I wholeheartedly agree that firefox needs to have two rollouts. One with and without extensions. The idea of having an application, with an appropriate disclaimer which says mozilla is not responsible for anything the third party extensions do or dont do, that lets you choose which extension you want installed along with firefox is amazing!
I currently sit on a standards committee for the school district I work in and we shot down firefox, even though many of the admins use it on thier machines themselves, because of several problems we saw as user issues with the browser.
Some of the other things we saw problems with were:
No Active X support (many of our online applications use active X)
Not as user friendly as other browsers (ease of use and clarity issues)
Lack of a real centralized support center (The forums are a rich resource..if you have time to run searches or wait for someone to answer your post, which in a real world environment, is not conducive)
Potential for abuse by students of all age ranges (The tabbed browsing is an exceptional idea! however, most teachers are too sued to window browsing and wouldn't even notice the extra three or four tabs that are in the background hiding god knows what kind of sites from her view.)
We really REALLY wanted firefox, but these issues just couldn't be countered. Everytime we tried to see if there were possible solutions, we were either met with hostility on the forums for daring to suggest that firefox was lacking in any area or we got silence.