Did you recently install Internet Explorer 10? It forces the installation of a Windows "update pack" that makes quite a few video changes. Some users have reported that it mucks up text in Firefox (among a slew of other complaints about problems that the update is causing).
I had a similar problem, and I thought my eyes were failing!! It popped up out of the blue, then I remembered a recent windows update to windows 7. I followed the above link and followed the directions, changed firefox's config and restarted firefox. Problem solved!! Thank-goodness it's not my 67 year old eyes after all!!