Firefox updated itself to version 32 and my ability to turn the forced high contrast mode for pages when Windows 8.1 is running in HC mode disappeared, even within a sandbox as worked previously.
I compiled version 32 from source and applied the patch to use an extra configuration variable as detailed here. It worked but, as I found with previous compilations, it consistently refused to load Adobe Flash (the plugin crashes, leaving Firefox running). This would make it annoying to use as my mainstream browser.
I guess the crash is due to me using Visual Studio 2013 when the official compiler is documented as Visual Studio 2010. Or soemthing else. Its been that way since version 28 and persists with the latest nightlies and build environment.
It occured to me that the problem causing the Flash crash might be in a different DLL to the fix for the high contrast override, which compiles to XUL.DLL.
So I copied my compile of XUL.DLL to a stock release of version 32, and I now have both Flash and my colour override working. Good times are back.