Numerous web browsers have already stopped supporting Windows XP and Vista, but not Firefox. To this day, Mozilla's latest software can work with your decade-old PC. However, even that team has its limits -- it's phasing out support for XP and Vista starting next year. Mozilla will start by moving users on these operating systems to the Extended Support Release in March 2017, limiting them to feature updates that can be "several cycles" behind the curve. And while the company plans to unveil a final support end date in the middle of that year, it'll effectively cut the cord in September, when it stops delivering security updates.
This doesn't mean that Firefox will stop working on XP or Vista in September, of course. But when security flaws are hard to avoid, it'll be impractical to keep using Firefox on those platforms when you could remain permanently vulnerable to exploits. There aren't likely to be many people who both run a 10-year-old operating system and care enough about their web experience to use a recent browser, so the impact may be limited. However, it's still a big deal if you're stuck on a work PC or otherwise haven't had a chance to upgrade to a newer version of Windows. Like it or not, you'll probably have to consider a new OS (or a new PC) if you want to experience the modern internet next year.