If Mozilla really wants to save themselves some time, they should consider slimming Firefox down. They need to stop developing features no one wants in the first place. EME, and Pocket are both great examples of unwanted features, and are the reason I'm using Abrowser instead of Firefox.
As for Windows XP and Vista, that's going to be a real problem for many users. I know a handful of people who haven't upgraded from XP yet, and need modern browsers to browse the modern web. I also use Vista every once-in-a-while, mostly for Windows-only games, and occasionally need to browse the web for one reason or another.
Thank goodness GNU/Linux upgrades tend to be so much easier then Windows upgrades (not to mention free), since they sometimes have much shorter lifespans than Windows.