Probably the plug-in is "Ad Block Plus 0.7.5.1," because that's the only one I am runnng. However, that plug-in is rather key to a non-payer like me enjoying FT (although maybe of lesser concern to you folks, all things considered!).

Edited to add: Hmmm, I just noticed that this only happens SOMETIMES. I just posted a response in TB topics where it didn't happen. So it almost certainly is related to a specific ad that Ad Block Plus is somehow interacting with weirdly, which means that in time, this too shall pass.

So probably the easiest way to handle things for now is do nothing and see if it goes away in a week or two.