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.
That’s the thing, this type of change is easily bypassed and therefore irrelevant to the vast majority of the people that even would think of switching to Linux when faced by that annoyance.