Remove an ugly outline inside the buttons in Firefox

packages/@uppy/core/src/_common.scss

@@ -146,6 +146,10 @@
     cursor: pointer;
   }
 
+.uppy-c-btn::-moz-focus-inner {
+   border: 0;
+}
+
 .uppy-c-btn-primary {
   font-size: 14px;
   padding: 10px 18px;