The instance of Firefox running on my machine just stopped responding to events. Pressing a button in a dialog, or even closing the dialog, now has no effect. So now I've got to manually note down any open pages important enough to revisit, kill it and restart.
The browser has been prone to doing this since it was Netscape Navigator; not to mention its memory leaks and tendencies to churn and use up CPU cycles. You'd think that a technological civilisation capable of putting a man on the moon would be able to design a web browser that works properly, but no.