This graphics glitch is a known bug in Firefox running on Windows 8. It appears to manifest only on machines that run on nVidia graphics cards and it is caused by the browser using hardware acceleration.
The fix is simple – disable hardware acceleration from your browser’s settings menu and the strange colors will be gone – and so will be your browser’s hardware acceleration unfortunately. But that’s all we can do until the bug gets fixed.
You can track the issue on bugzilla here: https://bugzilla.mozilla.org/show_bug.cgi?id=686782
And just in case you can’t find the menu where to disable hardware acceleration: open the options window (Firefox – Options or Tools – Options) – Advanced – General – and here uncheck the ‘Use hardware acceleration when available’ checkbox. That’s it.