Unfortuately, that didn't work. Strangely though, when it happens in one browser, it seems to affect other browsers as well.
I had a thought… because this is on my laptop, I sometimes use it behind a corporate firewall and that's when it occurred today (after being fine for the first few hours behind the same firewall)
Is it possible that this is only occurring for you at a school or workplace?
Edit: Seeing as I'm home now I've started to load up my sites again, and they're all working. I think there's a good chance it is the corporate firewall getting in the way, and that Firefox might have been running off cache for the first few hours.
I suppose that's why the browsers I don't usually use as much (Chrome and Safari) didn't show the themes either - they didn't have the cache stored for those pages or had auto-deleted the cache when I logged out…