Done. I have added XUL-based support for OS X Touchbar to Postbox and that will certainly ship soon. It's then probably the first Gecko-based application with OS X Touchbar support... All in all, it was quite easy. I only regretted some very strange or sometimes inconsistent design choices on Apple's side. But let's get back to the results... In the code, we have for instance:

code in Postbox's main window

And the result is:

Postbox's main window and touchbar

All in all, quite cool, simple to understand, manipulate, and even extend (on both the XUL and Cocoa side). Was fun to implement :-)