Company trying to make money off of its fork.
> Vivaldi
Company trying to ???
> Edge
Microsoft, who found that maintaining a chrome fork would be less expensive than playing catch-up with their own in-house browser.
Examples of this are the Electron Framework [0], Vivaldi, Brave, Opera, Yandex, Edge, etc.
Firefox instead is a nightmare to fork. They used to have something called XulRunner[1] that allowed to create your own XUL application (things like Seamonkey, Thunderbird used it) thus making it fairly easy to fork Firefox. After the 41 release Mozilla removed it completely. XulRunner's components were intertwined with Firefox code. Mozilla deliberately killed the easiest way to work their product.
Only light forks like Waterfox, LibreWolf are viable. Hard forks fail or struggle every single time Mozilla releases a new version (SeaMonkey, Waterfox Classic, Pale Moon, etc), lagging behind in features and performance.
Even WebKit is easier to integrate with your own UI (Safari, Gnome Web [2], etc).
[0] https://en.wikipedia.org/wiki/Electron_(software_framework)
The choice between forking Chromium and Firefox is mainly one of business[0]: Chrome has a >70% global marketshare, adding Edge & co even ignoring Safari it's probably around 80. Since Google also keeps pushing their own stuff, that means forking Chromium gives you much better compatibility guarantees.
[0] though the history of Chromium — and Webkit before that — forks also means there's probably a lot more knowledge floating around about maintaining such a fork, especially since Chromium itself was originally a fork (running concurrently with its source and regularly synch-ing from it, forking a dead codebase or hard-forking with no sync is a different concern)
Firefox forks tend to dislike associating with any of the above.
It's still not comparable for a fairly simple reason: the list of companies in the world that are as big as Microsoft consists of Google, and Apple, both of whom already have their own browsers.
As for why Microsoft chose Chromium, it's probably a combination of marketshare, the fact that it is a bit more cleanly architected as a result of having a decade less history than Gecko does, and the fact that they have ambitions of making a stripped down version of Electron part of the standard Windows userspace.
Options:
1. Fork Firefox, people install Chrome anyway 2. Fork Chromium, some people realize that it's essentially the same as Chrome and don't install Chrome and just use Edge
Also, especially on mobile, Firefox is an extremely niche browser engine. The biggest browser forks in therms of global user count are actually not the likes of Edge, Brave, etc, but android Chromium forks popular in asia.