I am also having this problem. I spent the day troubleshooting and researching it. Resetting the FF zoom did nothing, unchecking hardware acceleration did not, adjusting clear text in Windows did nothing, changing the font used in Ff did nothing, using a clean FF profile did nothing. The only thing I have not tried is a complete uninstall of Norton Internet Security. Although Norton is working just fine on Windows 8.1 and is fully updated, I wonder if it is an issue. Probably not though, because Firefox has had the same problem off and on since version 3. I have been unable to find a valid cause or fix online. I cannot upgrade the video card drivers or I will lose my ability to play Adera, but the drivers worked fine before the 8.1 upgrade. IE and Chrome are not having this issue, nor is the general system. I found an add-on called Anti-Aliasing Tuner that I may try next. Since it is an older add-on, some editing of the .rdf file may be necessary to get it to work on FF 24. I will post back if it works or if I find a solution.