Killing XUL is not a problem in itself, what's not clear is whether WebExtensions will support the same advanced features, and the goal of keeping compatibility with Chrome implies it won't.

Chrome compatibility isn't a major goal, IIRC. WebExtensions are based on the Chrome API but will neither necessarily implement all Chrome features, nor be restricted to the set implemented by Chrome, AIUI.

Well, it sounded like a major goal in their announcing blog post:

We would like add-on development to be more like Web development: the same code should run in multiple browsers according to behavior set by standards, with comprehensive documentation available from multiple vendors. To this end, we are implementing a new, Blink-compatible API in Firefox called WebExtensions. Extension code written for Chrome, Opera, or, possibly in the future, Microsoft Edge will run in Firefox with few changes as a WebExtension.

https://blog.mozilla.org/addons/2015/08/21/the-future-of-dev...

Killing XUL is actually a significant issue. It affects:

-Thunderbird

-Instantbird

-Nightingale

-Celtx

-Blue Griffon

-Pencil project

-Komodo IDE

And others.

Fair enough, I meant to Firefox itself.