This bug is so irritating, I believe it is enough to stop a LOT of people from using Firefox and Ubuntu altogether. Too bad. This had to happen just when I was starting to like really like Ubuntu and very close to recommending it to my friends.
As I commented in the bug page, this seems to be not a problem with the flash plugin but more like an issue in Gecko, the underlying technology used by Firefox and other Mozilla-based browsers (Epiphany for example).
The exact same plugin works without problems in Opera, which doesn’t use the Gecko layout engine.