Long story short: That pop-up you get when you open Firefox (or whatever other browser) for the first time asking you if you want to make it your default browser won’t work anymore. They’re forcing you to go through their convoluted control panel where you have to change it like 20 times for each file extension.
It’s not just those files in Windows 11’s case, it’s things like shortcuts you have on your desktop, clicked links, and a bunch of other things which all get defaulted to Edge and you have to go in and change all of them.