I don't think that when -moz-appearance on an <input type=range> with default styling is flipped between "none" and "range" that the bounds of the slider that is displayed should change [much] - it should pretty much just change how it looks. Specifically, the length of the range's track shouldn't pop out to be longer/shorter when -moz-appearance is flipped between "none"/"range". This mostly implies that either there should be no CSS border or padding by default (and in order for -moz-appearance:range to apply native theming), or else we'd have to draw the native theme into the range's content box instead of its border box. The latter solution is incompatible with the way that we currently draw native theming using nsITheme::DrawWidgetBackground (given that that fills the border box, not the content box), so the former solution is the only reasonable solution that I see. If we decide on the latter solution, then forms.css should at least have a comment added noting the border/padding requirements and rational, and that any positioning adjustments will have to be done with margin.
Actually the theming code has borders/padding logic that I didn't notice.