fix(sort): extra Firefox focus outline not being reset (#14733)

Fixes the extra outline that Firefox adds to focusable buttons not being reset for the sort header buttons.

Author: crisbeto

src/lib/sort/sort-header.scss

@@ -31,6 +31,13 @@ $mat-sort-header-arrow-hint-opacity: 0.38;
   outline: 0;
   font: inherit;
   color: currentColor;
+
+  // The `outline: 0` from above works on all browsers, however Firefox also
+  // adds a special `focus-inner` which we have to disable explicitly. See:
+  // https://developer.mozilla.org/en-US/docs/Web/HTML/Element/button#Firefox
+  &::-moz-focus-inner {
+    border: 0;
+  }
 }
 
 .mat-sort-header-arrow {