It's a great browser, but I always forget the default settings are super stupid. Myself and power users all have it customized to the hilt.
It takes some serious work to get a new new FireFox install working nicely.
But it should go even further; the ultimate goal should be for all Firefox users to basically look the same from the point of view of third parties and put an end to tracking in the modern Web.
How much do these break functionality? If I spoof language, am I going to start seeing websites in German? If I spoof screen size, am I going to get weirdly zoomed websites?