For a number of years we've held off on updating our Windows toolchain to a newer version of Visual Studio, and in so doing preserved support for Windows 2000 and Windows XP RTM and SP1. Firefox developers and the 99.6% of our Windows users have paid a price for this support, though. Our developers have not been able to take advantage of new compiler features and have had to struggle to keep valuable optimizations from breaking -- including having had to back out and ultimately delay some important new features like SPDY. Our users have have suffered a slower Firefox than would be possible as both direct and indirect results of moving to a more modern compiler.
So this week, after a few months of discussion and evaluation of the latest Firefox user numbers and the pros and cons of moving our tools forward, I've called for Mozilla to begin the process for ending support on those older Windows version. Next Tuesday or Wednesday, after Firefox 12 moves to Aurora, the Mozilla Release Engineering team will begin upgrading our Windows build systems to Visual Studio 2010. With VS2010, we will no longer be able to build a Firefox that runs on Windows 2000, Windows XP RTM, and Windows Service Pack 1.
It's always a difficult decision to leave some users behind. The number of Firefox users on those OS versions -- less than one half of one percent of our Windows Firefox users, and the benefits to our development process and the hundreds of millions of Firefox users on XP SP2 and above, however, compel us to look forward rather than back.
If you are a Windows 2000 user, Firefox 12, which will be supported until June 5th, will be the final supported Firefox release. After that, your options are limited. Switching to Opera is probably the best path forward.
If you're a Windows XP user still on RTM or Service Pack 1, I strongly urge you to install the free Windows Service Pack updates.
And finally, for Enterprises adopting the ESR, these older Windows versions will be supported for the length of the first ESR of Firefox. That works out to an extra 6 months or so before these Windows versions become unsupported.