After upgrading to Windows 8.1, I started to notice that the fonts in Firefox would occasionally go fuzzy and unreadable. This would usually happen after scrolling the page up or down a bit. I actually had to switch to Chrome while I researched a solution to the problem.
It turns out that this problem is related to Firefox’s new rendering engine, code named Azure. Apparently the good folks who maintain Firefox have completely rewritten the rendering engine with the goal of making web pages draw faster. All well and good but it seems as though there are still a few kinks that need to be worked out. Anyway, the solution to the problem is to disable use of Azure and go back to the old rendering engine. To do this you need to manually edit to Firefox configuration.
Go to the configuration page by entering about:config in the URL edit box. When the list of options appears, search for gfx.content.azure.enabled and change its setting to false. That’s all there is to it.