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.