Edge, for example, is a fork maintained by Microsoft. It is a strategic project for a multi-billion company. That is not comparable to a fork of your average open-source project.
>But it's absolutely comparable to a fork of Firefox.
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.
It was definitely a strategic business move. Chrome is eating everyone's lunch with marketshare.
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