I understand why Firefox is doing this, they're not the only one. But I also understand why people would like to stay on older operating systems, particularly Windows 7.
With this, Mozilla will have given more than 1.5 years of extra support compared to Chromium browsers. Supporting old operating systems has a development cost, and they put themselves at a disadvantage if they choose to support these old OSes for much longer.
People should really just update. Old, unsupported operating systems are unsafe to use.