Using firefox nightly, but couldn't find any nightly-specific feedback mechanism. I have this problem regularly, where Firefox will fail to render a tab at all, and then rendering for that tab is completely busted. Usually, any new tabs I make shortly after will also be busted (i.e. have this same issue), but eventually I'm able to create new tabs that behave properly.

Any tab can get in this state -- it happens with facebook, reddit, google, small websites, etc.