Yep, but it seems to be a PITA. They endure whatever changes happen on Firefox UI, which are well tested on Firefox, but not on Thunderbird and Thunderbird has much more UI to manage than Firefox. See this interesting Thunderbird talk at FOSDEM on visual change that mentions this issue [1].

You also kinda have to fork Firefox to do this. It would be good to be able to #include <gecko-embedded-framework.h> and build the UI from there. XULRunner seemed nice too.

Using Gecko when you are not Firefox is such a pain that

- all alternative browsers that are not forks of Firefox that were based on Gecko have abandoned: they stopped being maintained, or switched to WebKit or Blink, which is a shame.

- all apps based on XUL / Gecko, like Songbird, have mostly disappeared.

It needs to be easier.

Gecko seems like a drag for Thunderbird. It shouldn't. For this, it needs to be a proper toolkit, with stability guarantees, and proper support to third party apps, and easily reusable. That's not the focus for Firefox devs though.

[1] https://fosdem.org/2024/schedule/event/fosdem-2024-2728-thun...