My signature-uncluttering phpBB Firefox extension seems to have been listed recently at Extension Room. And as proof of someone already using it, a few hits in the access log show that browsers are eagerly polling for updates -- and I received a second feature request, not counting my own. (With a bit of luck, I'll learn how to get a callback once just the HTML bits of a loading page have been fully loaded, so I can address my own issues with it. After all, it doesn't make any sense loading lots of graphics just to purge them a second later.)
Macromedia security team got back to me reporting that they have reproduced the problem and that it fortunately only feeds mouse events to the background flash application (not including data on which window should have seen the events). I hope the Macromedia and Mozilla people get to share intelligence on the subject; I'd be delighted if the former nailed their bug and the latter found a way of fixing the entire problem class from their side. Since IE for once seems unaffected, I have hope it's possible to somehow alter the plugin interface sandbox to catch this.
Oops, I just encountered a solution Net doesn't consider valid. Minesweeper addicts beware: this game carries the same mind tying properties that your former brain virus did.