[andrewroff]

There is SeaMonkey - https://www.seamonkey-project.org/

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.