If you haven’t upgraded to Firefox 1.5, then you may still be dealing with those horrible dialog boxes when there’s an HTTP or DNS error. The new version has nicely formatted standard pages, like the sample above.
I always hated those dialogs, because when a domain wasn’t found or something, you had to stop what you were doing and dismiss the box. Whenever I realized that I typed a domain wrong, I’d have to race the browser to correct the address and hit enter before the blasted box popped up.
There’s an extension to stop this behavior and revert to IE-style standard error pages, if it drives you crazy enough (can’t find it again — if you know where it is, do tell). Or you could just upgrade.