Mozilla had an Electron alternative before Electron even existed. It stopped being a viable option several years ago, though. The reason is the same as for every other problem that Mozilla is afflicted with: incompetent leadership.

(Note also that the choice is not a dichotomy between Blink and Gecko. WebKit exists.)

Webkit has the same foundation as Blink. So while it's diverging they are much closer than Gecko or Edge HTML.

To say "it's diverging" has to be the understatement of season. Two simple questions for you: what was the purpose of your comment, and how is it relevant in this context?

I know there are browser fans who have a preference of UI or companies etc etc, but the answer to the question of why so many people choose to fork Chromium to make their browser distro is solely based on the fact that it's technically the best browser engine out there and to fork anything else would be putting you at an immediate disadvantage compared to forking Chromium, so you'd start with the best and then extend it from there.

That's basically what Brave CEO Brendan Eich said when asked why chromium on a podcast.

The last time I looked into this (about 3 years ago), it was ridiculously difficult to embed Gecko into anything, almost to the point of being impossible. Chromium in comparison is relatively easy to embed (via CEF or LibChromiumContent).

All I can tell you is that when watching HTML5 videos on Firefox my CPU spins way up. On Chromium it does not.

Also yes in general every JS-heavy web app I've ever used runs much smoother on Chromium than FF.

I don't have a real answer for you, more just shades of understanding. I believe it's the case that Chromium is just more extensible and designed for reuse. Whereas Firefox is more monolithic and less able to be extended.

I don't think it has anything to do with the javascript engine per se, but more the general API surface that Chromium gives you. There might be some additional benefit that Chrome is the most used browser (currently) and so there might be some developer sentiment or familiarity with it; Chrome's popularity might push adoption for Chromium.

I don't understand why Mozilla itself doesn't support an Electron clone.

Because the people calling the shots don't understand the "market". They've been coasting on their (predecessors') early success for years. Their numbers with respect to audience size is dependent on the same factors that OpenOffice depends on: people with a faint awareness of what the project is / once was but no real exposure to the ins and outs of what has changed about the projects in the last several years.

Short answer is that the amount of time and energy to create and maintain it wouldn't be justified by the expected gain.

I'm not even sure what moz would gain here.

I can see a lot of value in having their technology more widely distributed.

The same could be said of the browser tech itself. They could just switch to a webkit engine, have the same market share, and still "gain" whatever it is they are gaining now.