Seems to work for me right now, Firefox 52.0.1 on Win 7.
The comment piqued my interest, and the result was an eye opener. Didn't realize anyone still used Win 7, but this article says it still has a 49% market share.
What, me worry?
Well, when I bought that laptop just around 2 years ago, it was that or "iPad 8". Not wanting my PC to look/work like a darn tablet (if I want a tablet, I'll buy one!), I specifically bought it with Win 7. (So from my perspective, it would be "I can't believe anyone actually bought a PC with Win 8 on it."
Win 10 was out shortly after during the "free" upgrade, but I never did it due to the many people who had horror stories of what happened to their systems after they tried to do so in the early days of the "free" offer.
I personally think operating systems need to ripen before they are reliable.
-Dave