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.