It's essentially a continuation of the Mozilla Suite (formerly Netscape). However they're falling behind on Gecko integration, with the latest version still on Gecko 60 (2018) with backports.
The reality is that Mozilla doesn't focus on embedded applications of Gecko besides Firefox (even Thunderbird is sidelined these days). Projects that are essentially third-party, like SeaMonkey, really have no visibility at all. This is the reason why the Chromium Blink engine has become so widespread, especially when it's been nicely packaged up by the commercially-backed Qt project.
I don't know why there aren't more browsers based on Gecko; especially since there are quite a few people who are not exactly thrilled with the general direction Firefox has been going.
There's PaleMoon, which is a fork, and there's SeaMonkey, although the latest version is based on Firefox 60.8 (May 2018), so I guess that's more of a fork than "using the Gecko engine" too? Also the UI is a bit too 1999-esque even for my tastes.
So I guess it's just hard to re-use Gecko? Using QtWebEngine (or QtWebKit before that) is very easy. No really, it's 20 minutes and you have a basic browser. I built a basic "vim-like" browser in the style of Qutebrowser in a day with PyQt (missing a lot of UI stuff and polish, obviously) and I'm not even very experienced in Qt/PyQt (or GUI programming in general).
Yes, Firefox/Gecko is much more one thing, Mozilla apparently even had some problems with that close integration and baked-in assumptions when building the Android variants.
Splitting this out a bit more seems like it would be a good strategic choice, not just for Mozilla, but for the web in general. There are undoubtedly all sorts of technical challenges involved, but betting on only Firefox is putting all eggs in one basket.
Say what you will about Blink/Google, but the easy with which you can build a browser on top of it has spawned a plethora of browsers, ranging from Edge and Vivaldi, to all sorts of more niche ones like Otter and Qutebrowser, and many more.
Half of Vietnam is using Cốc Cốc. Yandex Browser is fairly popular in Russia, UC Browser is popular in China (or rather was, since it was banned) and some other Asian countries.
There's no reason at least some of these couldn't have been a Gecko-based browser.
Yeah, for smaller projects with only a handful of people behind them, WebKit (via WebKitGTK) or Chromium (via QtWebEngine, Electron, Chromium Embedded Framework) are pretty much the only choices there are. The only exception maybe being Pale Moon.
For browsers which are based directly on the Chromium codebase without being supported by Google, there are definitely many more resources needed - I don't see a particular reason why those couldn't be based on Firefox either. From what I see in QtWebEngine, keeping up with Chromium isn't exactly easy - IIRC they said they need about a person-month to catch up with a new Chromium release, usually with millions of changed lines.
I presume part of this is that for a long while the KDE/webkit/chrome engine was a separate, isolated thing, whereas the rendering in Firefox was not.
Not sure where Firefox/gecko is now, but pulling the rendering apart from the UI and improving it to be usable by other UIs, projects, would help get this done.
It's essentially a continuation of the Mozilla Suite (formerly Netscape). However they're falling behind on Gecko integration, with the latest version still on Gecko 60 (2018) with backports.
The reality is that Mozilla doesn't focus on embedded applications of Gecko besides Firefox (even Thunderbird is sidelined these days). Projects that are essentially third-party, like SeaMonkey, really have no visibility at all. This is the reason why the Chromium Blink engine has become so widespread, especially when it's been nicely packaged up by the commercially-backed Qt project.