Firefox will likely remain supported on XP for even longer than Chrome. Firefox was still supporting Windows 2000 when Chrome was originally launched... at which time Chrome only ran on Windows XP or better. At the time of Chrome's launch, Windows 2000 was still within its support lifecycle and would be for another 2 years.
Firefox 12, released just last year, was the last version to support Windows 2000 despite the fact that Windows 2000 was end of lifed in 2010. If they stick to the same old OS support timeframe, Firefox would support Windows XP through 2015.
Firefox 3.6.28 and Firefox 12 are the last versions of Firefox that will work with the original version of Windows XP and Windows XP Service Pack 1. They won't be updated with security and stability fixes. In order to keep your computer and personal information safe, we recommend that you upgrade for free to Windows XP Service Pack 3 and install the latest Firefox.
I was doing an install of a bunch of computers for a local company's call center in 2009. Old throw-away machines that didn't need internet access but needed a browser to view the intranet and play media files (they did phone surveys for radio stations and would play music over the phone). I was brought on for my Linux experience, but there was one box out of 28 that we could not get working with Linux for one reason or another. They had me install Windows 98, and the only browser that would play media from their intranet on Windows 98 was Firefox 2.x (I can't remember the exact version).
It wasn't a fun experience, but color me surprised that Firefox 2.x would work on Windows 98.
