A new interface introduced in Windows 11 made changing the default programs a lot more difficult. That also includes changing the default browser.
Before Windows 11, changing your default browser was relatively easy, all user needed to do was clicking on your browser of choice and selecting it as your default browser.
However, now users have to search for each file extension (.html and .htm) and protocol (HTTP or HTTPS) and manually associate them to the program they wish to use.
Mozilla has managed to reverse-engineer Window 11's default settings process to allows users to switch their default browser with one click.