fix(tabs): Disable firefox focus ring (#4560)

Author: williamernest

packages/mdc-tab/mdc-tab.scss

@@ -50,6 +50,11 @@
   cursor: pointer;
   -webkit-appearance: none;
   z-index: 1;
+
+  // Firefox still draws a dotted border around focused buttons unless specifically overridden.
+  :-moz-focusring {
+    border: none;
+  }
 }
 
 .mdc-tab--min-width {