The only upside for Mozilla is that making Gecko available is the right thing to do. Which, for a nonprofit, matters, and I'm sure that having an Electron-equivalent is the kind of thing that would be considered cool by staff.

But there is the downside that it makes it easier to build browsers which would compete with Firefox for the shrinking niche that is "doesn't run Chrome or Safari". I guess you could make a case that such browsers would take at least as much share from Chrome as Firefox, maybe more even; but you could make the other case as well.

When you add the additional downside that it's probably many man-decades of work to do it at all, well, then it doesn't happen. Although the existence of GeckoView implies that Firefox and Gecko aren't as tightly coupled as they could be.

Chrome team was advcied very early in their development by none other than Android team to go for WebKit because it was easier to work with. Not Presto, not Gecko. So at least since back then, thud feature was at least somewhat on Apple's list of priorities. I'm not sure if it got carried over from KHTML, but if it did, they didn't break it intentionally.

Back in IE horror days I remember using Avant Browser. It used IE to provide fancier features like tabs. I never remember Gecko anywhere in discussion for being a platform to build on top of, except for other Mozilla projects (FFOS, Thunderbird) that Mozilla dropped sooner or later.