Mr. Wizard,
a few things I would try.
1. clear the cache in firefox (and I'm assuming it's the most current version).
2. Try a different browser and see if it renders 'normally' in that one (doesn't Win 8.1 have IE still as part of the Operating system?).
3. let us know if it clears it out for you.
I'm using firefox on a macbook (2010) and it's working fine. My gut tells me that somehow you danced on your keyboard and changed something in Firefox. There is a full reset of firefox you can do as well to help 'reset' everything as it should be.
josh