Windows 10 Chromium Edge cannot be uninstalled and one main reason is that it is the built-in viewer for all Help System displays.
So no, you cannot change this or use Firefox for the help browser. Edge is built right in.
Edge will continue to be used for extensions of Help (because Help will be calling the extensions). By extensions I mean Web help called by Help.
I will look for an article, but it falls in the realm of common knowledge as Microsoft issues its updates.
I trust this explains it.
Anecdotal Note:
My time with Windows 10 has been 6 years including Previews in early 2015. I have 2 production machines 20H2 up to date and 1 insider machine 21H something new. Help in Windows 10 has been built around Edge all that time.
Help in Insider is progressing forward from Help in 20H2. Chromium Edge is being more tightly integrated and it shows up in overall change in looks and the extensions into Web Help (different browser windows that do not affect my own Chromium Edge normal setup.
I read the Insider Notes and they generally support the above.
I have had the very good fortune to attend sessions in Redmond WA over several years (non-disclosure of course) but the presentation about the use of Edge supports the above as well.
So Help is a Chromium Edge world, and default browsers of our choice are for web work outside of Help. Of course there is a grey area going from one to the other.
Again, I trusts this helps you.