You do know, Mozilla could recreate the old experience (Firefox 20-22) which offered incredible functionality in the core and offered both advanced and not so advanced users the best of both worlds.
Just take a look at Vivaldi’s Non native UI - With JSS and Javascript it would be without trouble possible to recreate all the features Mozilla has deleted, and even theming would be similar possible like it is right now. Statusbar - no problem, All UI elements movable - also not a problem, double row tabs - also only a matter of code - You see, the options are almost without limits.
The only reason that someone is starting to talk about API is if it is only partially planned to recreate the feature set which is going to be removed. When Mozilla switches over to a more native UI if i compare it with the XUL system right now, much more features are vanishing… Userchrome.css anyone?
A negative example was Opera new - Switching over to Chromium with it’s native UI (btw. here are much similarities between what Opera did and what Mozilla plans) forced Opera to give up all advanced features and the API’s they have been creating afterwards which have been introduced to offer some kind of “feature replacements” are rather limited and similiar to Australis and beyond, the advanced users would be put in responsibility to bring features back… again!
Anyway, the point is that: it would clearly be possible with the help of certain technologies to bring back a browsing experience which works for both, tech users and non tech users, but the thing is, Mozilla is not interested at all in doing that anymore.
And yes, i understand the need to make Firefox more attractive for non tech users, as they are the majority of users. But on the other side, why not hidingall advanced features behind some switches which can be activated and deactivated… a so called expert mode? About:config is a good enough solution to keep non tech users away from advanced functionality!