This isn't a problem, it's just an extra feature/option provided by the browser OP is using. You get the same dialogue if you are using Chrome but without the option to disable JavaScript dialogues until the next browser launch.
Several times? Where? Can you please show us? And, how can a bug create a checkbox followed by a logical question in a dialogue box?
As I said above, it's not a problem that needs to be solved, it is just an extra option provided by the browser being used, probably, and clearing the data won't make a difference here.