Not only that, but I'm also concerned about the future of addons like Pentadactyl [0] and Vimperator [1] that use XUL.

I know the user base of these addons is not anywhere as big as Thunderbird, but I can't really imagine using Firefox without Vimperator; and it's already started to break in Firefox 45.

[0]: https://github.com/5digits/dactyl

[1]: https://github.com/vimperator/vimperator-labs

I'll jump in and mention VimFx [0] here because, having the same concerns, I recently switched over from vimperator. It's lighter than vimperator but gives much of the similar functionality, without completely revamping the browser. And it should continue to work as Firefox moves ahead.

[0] https://github.com/akhodakivskiy/VimFx

VimFx (and Vimium for Chrome -- XUL deprecation and the loss of Pentadactyl migth be what finally pushes to switch from Firefox to Chromium) are nice addons but feature wise they do only a small subset of what Vimperator/Pentadactyl do. I use Pentadactyl and I get a lot of use out of writing my own plugins, using autocommands to trigger different actions on different pages, using quickmarks, and using ":js" on the command line to debug javascript code.

I plan to use VimFx/Vimium in the future though because those keyboard patterns are ingrained in me now. Pentadactyl development has dried up over the last few years and it seems unlikely that such a huge change as XUL deprecation will be addressed. The developers have been asked about this several times through different media and not responded. I'm not sure about Vimperator. The developers are still active but porting is a lot of work and currently all of the necessary API's do not exist. Hopefully, I can cobble together something resembling Pentadactyl/Vimperator by using a few other extensions in addition to VimFx/Vimium.

  but I can't really imagine using Firefox without Vimperator

I can second that (s/vimperator/pentadactyl/). pentadactyl at least still works in Firefox 45.

Come over to Pale Moon [https://www.palemoon.org/] (firefox fork continuing XUL) and FossaMail [https://www.fossamail.org/] (thunderbird fork based on Pale Moon Goanna engine).

These products aren't perfect, but they allow the traditional strengths of firefox (powerful extensions based on XUL/XPCOM) as opposed to the new chromification of the firefox extension api. Also, you don't have the mozilla politics and constant moral hectoring (if you read the firefox start page or go to mozilla's website, I think you might know what I mean).

I don't personally have a problem with Mozilla trying to defend the open web. If anything should be their mission, that's a good one.

And while XUL is the base that many extensions have been built on, Mozilla are quite right to throw it out. If Firefox chucks it, I doubt forks of it will manage to maintain it well. It's half-dead already.

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.

Considering that Pale Moon strips out accessibility in the first place, I'm not sure an extension designed to increase keyboard accessibility would be the best fit.

Frankly I am pondering going back to Seamonkey (what used to be the Netscape/Mozilla application suite).

SeaMonkey likely has even fewer resources to maintain the mail engine within than Thunderbird does.

I use Seamonkey as my newsreader, works pretty well.

Yes, this is why the issue has become urgent right now. Thunderbird barely has the resources to keep up with the underlying changes to Gecko with each release. It has nowhere near the resources needed to deal with the deprecation of XUL. I am grateful to the Thunderbird community for continuing to maintain a stable email client over the years, but it has been a long time since a new release has brought any new feature that impacted me. From following bugs on some features I would like to see, I get the impression that the code base is old and full of overly intertwined components that make changes difficult. One example is this bug opened eight years ago about making it possible to write emails in a new tab rather than a separate window: https://bugzil.la/449299

I hope that once the Thunderbird code base is split from Firefox the churn for the developers will be reduced and they can spend more time on improving performance and adding new features. I do worry though that Thunderbird could become less secure over time without the Firefox team maintain security issues in XUL.

I find it hard to believe that another organization would be a better home for Thunderbird than Mozilla, but some of the language in the blog post and report is disappointing to me. Mainly, the points about Thunderbird needing to be self-funding and use its own infrastructure. If the code bases split and Mozilla focuses its advocacy around Firefox, surely that should be enough to unencumber Firefox and give it a (in my opinion slim) chance to remain relevant. Mozilla really values Thunderbird so little that it can't give it any resources at all? Even when it has all of the infrastructure needed by Thunderbird already set up?

Well, they either have to rewrite Thunderbird's UI, or maintain a fork of Gecko that still supports XUL. I have no idea which one of these would be less work.

My between-the-lines reading of this post is that this is really what is leading the split between firefox and thunderbird. Not enough people at mozilla care about thunderbird and so either they split and stay with xul or die.

Of course I don't know for sure (I don't even use thunderbird).

Other enterprises have built and shipped standalone applications built on XUL/XPCOM. Curious how the roadmap for XUL looks like in the midterm future.