Like some others I have always used FF since my XP days, and currently on my Win 7 Professional system.
I have zero problems with FF ... I did disable Java (as per their recommendations) and that resolved any troublesome issues that would creep up once in a blue moon.
I would urge caution on upgrading to Win 10 ... although there is lots of hype out there claiming how much better it is, be sure to check out the Forbes Tech pages and other reports on it first.
I am eligible for the free upgrade, but have decided to pass up that opportunity. Win 10 would require me to update older laser printers and my scanner, either with new equipment or new drivers. Win 7 works for me ... and if it ain't broke, why fix it? I don't need all the new bells and whistles ... and the headache of learning a new OS after just figuring out how Win 7 operates and where it hides files :lol:.
I removed the "GWX" files from my machine totally disabling the Win 10 upgrade. Those files came in with the last Win 7 updates and consumed 3.0 Gb of drive without my knowledge ... very sneaky Microsoft there ... and suddenly there was a new icon on my task bar. My brother-in-law accidentally clicked on that icon and spend the next 12 hours undoing the entire mess trying to return to Win 7.