https://github.com/mozilla/browser.html
Web standards are fully capable of rendering entire user interfaces now. GUI toolkits are just needless dependencies for web rendering engines.
Also note that the same extension support is landing in Firefox OS (we are working on the missing apis), and will let you install the same extensions everywhere.
I understand people mourning xpcom access. You can do fun things with that, but I've also seen my share of horrors when I was reviewing add-ons (let's override the http protocol handler, what could go wrong!).
I choose firefox because of the power of its addons, this is a huge issue for me as a user.
And yes, I know that you will be consulting with addon authors about api functionality, but i guarantee tons of stuff is still going to be lost...
Not to mention all the potential extensions that nobody has thought up yet that will no longer be possible and probably never will be. The ability to experiment and try out new ideas that nobody had thought of before is what gave us a lot of the extensions people use today. An API that tried to imagine all the possible features people might need and then locked down everything else would have prevented a bunch of the extensions that are popular today.
This problem exists for the web in general though (being a tightly controlled sandbox where only features agreed on by all the vendors are allowed). So I think Mozilla is probably institutionally incapable of understanding the problem, since it undermines the central mission of the entire organisation - which is to convert all end user software into software that runs in a vendor controlled sandbox. People who appreciate the creative cost of tightly controlled sandboxes probably wouldn't go to work for Mozilla.
In fact, I'm one of those developers, and the first thing I'd do in post-XUL Firefox would be try to create a Tree Style Tabs alternative.
