I seem to remember a lot of large institutions and techies kept using Windows 2000 for years after Windows XP came out.
I know of many large companies that stayed with Windows 2000 until it was clear that their only upgrade path didn't include Vista.
Might be a pain for IT to replace with Firefox, but is doable. I did this for a 60-year-old and then skinned it just like IE so the difference was barely noticeable.