Flagging the submission seems like the wrong approach - this is something Firefox needs to fix quickly. I believe some websites let you insert your own CSS in certain cases, and this could easily be abused...

There needs to be a limit on how much time can be spent trying to render CSS or a popup box asking if you want to close it similar to how javascript behaves.

works on my FF18 on Linux, but the animation center is on top of the window instead of the middle.