There is an issue with that site in that it doesn't cache the data which is very wasteful and slow since JQuery is having to actually fetch and presumably process the whole JSON every time. However, as you say it is only less than 80k.
I can't see a problem on mine and a quick check didn't reveal any other issues.
My first thought would be a addon since Firefox issues are nearly always caused by addons doing something they shouldn't or exhibiting unexpected behaviours.