Mozilla has ended support for Firefox on Windows 7 (and 8). In a couple of years, less and less websites will work on that last-supported Firefox version. Other cloud-related software like Dropbox will also remove support sooner or later. The more difficult it gets to use online services and the web on Windows 7, the more people will switch. It will end up in a small hobbyist niche like people who run Haiku or Serenity.
Unlike between XP and Vista, there isn't that much win32 API difference between Windows 7 and later versions. I'm sure in due time people will come up with DLLs to shim the few missing functions.
