I would simply navigate away from the page. But then again, I'm fairly fast and loose with my browsing so more conscientious users may want to close down Firefox the normal way and maybe make sure that the firefox.exe process terminates before starting it up again. You can, of course, mitigate the effects of this and most other attacks by running your browser without admin rights and/or in a sandbox.
What I do in addition to running Firefox with DropMyRights
in a Sandboxie
sandbox is to use the NoScript
Using these tools leads to a much safer (and quieter!) web browsing experience while still allowing sites I trust like Bleeping Computer to have rich content, movies, etc.
And by using DropMyRights and Sandboxie, any exploits that do
get through will run without
Admin rights and in a sandbox that prevents any permanent changes to my system.