Maybe they decided there's too much technical debt and they can't make changes to the browser as fast as they wish they could in the current framework?
Not 'broken' in a traditional sense but still a valid reason to rewrite.
Yeah, but there is a trade-off involved. If you spend 3 years rewriting your toolkit, then the time you will eventually save on further changes has to offset those 3 years, and that's very hard for most projects.
You need to support that claim. The people who actually work on Firefox have a list of reasons why they made this decision:
https://blog.mozilla.org/addons/2015/08/21/the-future-of-dev...
Why are we to believe that you are better qualified to make that call?