Keep wasting resources on CI, testing, coding shims for missing OS features, and making releases for the benefit of the 12 people worldwide who depend on the W7+Firefox combination?
There's no need to target specific OS versions. Yes, MS has added new APIs, but the old ones are still there and function perfectly fine; and chances are that the users on W7 are not going to care about any new features anyway, so if Firefox doesn't have the same features when running on W7 vs. a newer version, it doesn't matter.
I have written apps that will run on anything from Win95 to 11. A minimalist web browser happens to be something I've been working on too.
Microsoft's backwards compatibility its is greatest advantage, but only if you take advantage of it.