::-moz-focus-inner: remove unnecessary specificity

[mattbrundage]

Updated to use the same specificity as in Mozilla's user agent stylesheet (resource://gre-resources/forms.css)

Also, this ::-moz-focus-inner rule has no effect on the primary focus style of buttons, so we don't have to "restore" focus styles with a :-moz-focusring rule.

:-moz-focusring styles are set in resource://gre-resources/html.css

Reduced test case: https://codepen.io/mattbrundage/pen/LYZKNYJ