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.
Oh, does ESR not have mandatory extension signing? That is pretty interesting actually! I thought unbranded builds were the only way to go, but they lack automatic updates.