When I start answering a question on superuser, with Firefox 11 on OSX 10.7, then push the back-button, I get the following notice. It seems like a javascript-alert. The catch: neither of the buttons work. The only way to get out of there is to kill the browser. I've had this twice today. It seems like a javascript-bug. I cannot close the window, or switch to other tabs, although other windows keep on working.

When I start a new question, this works like it should. I can go back to the previous page, no warning.