Comment 2 for bug 381703

Indeed, oftentimes it is the "most common" button on the right, however, in the rest of the cases (and actually, the official guideline it seems) is actually "affirmative button on the right". In our case, Add should be on the right because it is more likely that the user wants to use that instead of cancelling the whole operation (low probability), AND it is the "affirmative" action.

On that note... shouldn't the Close button in that dialog be a Cancel button instead?