Thanks so much for the response!
It's probably my system then; I'm on an older version of Ubuntu (linux) one which is no longer supported by the community, and an older version of Firefox. Unfortunately I can't upgrade my OS yet; this older version is one of the few OS's that works with my online course platform.
It's been squirrely with some other web programs, but usually it only gets unhappy about flash content. I may have to drag out the ol' windows desktop!