We had about six years where Linux wasn't the unsupported red-headed stepchild of the Web. It was nice while it lasted.
Firefox is losing market share, so it's understandable that they want to avoid the following: "We notice you have an unsupported browser. Please download Safari or IE to view this content". That wouldn't really serve anyone's interests.
Unfortunately, this just pushes the problem onto Linux as a whole: "We notice you have an unsupported OS. Please use Mac OS or Windows to legally view this content."