I'm using Mozilla SeaMonkey, the offspring of Netscape Communicator and Firefox. I mostly use it for the built-in HTML editor.
This has been happening for a while, not just with the latest Flash Player.This
bump plays fine for me. It's one that I encoded.
If I had to guess I think it's an encoding setting for H.264 that's being used on the bumps before they're uploaded to the site that's causing the problem. Could be too many B-frames or maybe trellis.
Without seeing the encoder settings it's hard to tell.
I do know that YouTube dials back some of the more advanced features of H.264, which is why their videos tend to have greater playback compatibility across browsers and devices.