Is this something that happens? I haven't tried Win 11 yet, but on my Win 10 PC I set Firefox to the default browser long ago and it's stayed that way through multiple feature upgrades and the initial Edge installation.
In Win10, the setting for Firefox as default works, but mainly because 1) Firefox renews that every time you use it, in background (or so I understand - they somehow manage to set/maintain it without
you having to invoke the Windows default settings), and 2) whenever you go into Settings, at least in Win10 Home, there's a bar on the top for recommendations, and it ALWAYS says you're at risk by not using the default browser and should reset that unless you're using Edge.
In Win 11, at least, that bit of nag is gone. Still nags about MS 365, which I don't need because Onedrive and LibreOffice get along just fine together.
I do have to admit, though, that it's been a long time since Win10 reverted me to Edge on its own, outside of a "Fresh Start" that nukes all the installed 3rd-party applications. Even upgrading to Win11 maintained my defaults for browser (FF) and email (Thunderbird).