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.
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.