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.
Just wait until they introduce hard-coded, built in AI into WIndows 11, that's when the "fun" will really start.