[AppBar] Fix z-index issue on Firefox (#21063)

Author: pedrooa

packages/material-ui/src/AppBar/AppBar.js

@@ -48,7 +48,6 @@ export const styles = (theme) => {
     /* Styles applied to the root element if `position="static"`. */
     positionStatic: {
       position: 'static',
-      transform: 'translateZ(0)', // Make sure we can see the elevation.
     },
     /* Styles applied to the root element if `position="relative"`. */
     positionRelative: {