Y U no validate

In the "Add Security Exception" dialog makes "Permanently store this exception" checkbox unchecked by default.

It's been created thanks to Jakub Wilk's idea, who has also submitted an enhancement proposal so that the extension wouldn't be needed anymore, because it's an overkill.

Source is available from a GitHub repository. You can download the extension from Mozilla Addons or use a package from the official Debian repository.