You're completely missing the point, it's about Windows ignoring users browser choice and opening stuff in Edge anyway.

The problem is that the microsoft-edge:// protocol is used throughout the Windows UI and menus and will always open in Edge regardless of the user's preferences. If you click on a link in say, the Settings UI, it will ignore your default browser preference and open in Edge with no way to change it.

Even if you uninstall edge, you can't make windows stop trying to open `microsoft-edge://` links with edge. The default-program-settings page lists it but it's a cruel joke because it doesn't let you actually pick anything except edge; you can't pick a custom program like you can for every other protocol. There are corresponding entries in the registry, and those you can edit to point at firefox... but they are simply ignored. It's quite frustrating.

I wonder if they plan on using that protocol to do non-standard stuff that third party browsers won't support? That's the only technical reason I can think of for Microsoft to create a new protocol.

Another possible reason is that the user can load help pages or search results even if the primary browser is broken, has a corrupt install, etc.

That would go well with their embedded buy-now-pay-later collaboration.

There's no problem with microsoft-edge: existing. The problem is that it's extensively used by things built in to Windows, which isn't fixed whatsoever by creating a firefox: protocol too.