I'm really not having any problems with FF running on my system with Windows 8.1. There is the occasional flash player crash, I've always thought it was because of having multiple tabs up and such.
I have a system with 8.1, Vista, Windows XP and Ubuntu LTS. I have a W7 license I'm not using right now. I have lots of friends that are calling me with support questions so I'm now on my backup system that runs all of them. Actually this is my backup-system because my main system died but that's another story, I'll rebuild a new "main" system with the latest and greatest later.
I also fix hardware problems and major software things for friends who bring their system over, sometimes I'll just buy a part for one who is financially challenged and say "Oh I had an extra one" other times I really did have an extra part. This is pretty much in my circle of Christian friends but I'll do it for others as well.