Include a local copy of the arrow icon

From https://design.firefox.com/icons/icons/desktop/arrowhead-right-12.svg

proxy/webext/icons/arrowhead-right-12.svg

+<!-- This Source Code Form is subject to the terms of the Mozilla Public
+   - License, v. 2.0. If a copy of the MPL was not distributed with this
+   - file, You can obtain one at http://mozilla.org/MPL/2.0/. -->
+<svg xmlns="http://www.w3.org/2000/svg" width="12" height="12" viewBox="0 0 12 12"><path fill="context-fill" d="M9 6a1 1 0 0 0-.293-.707l-3-3a1 1 0 0 0-1.414 1.414L6.586 6 4.293 8.293a1 1 0 0 0 1.414 1.414l3-3A1 1 0 0 0 9 6z"/></svg>
\ No newline at end of file

proxy/webext/popup.css

@@ -27,7 +27,7 @@ body {
   display: block;
   position: absolute;
   top: 12px;
-  background-image: url(https://design.firefox.com/icons/icons/desktop/arrowhead-right-12.svg);
+  background-image: url(../icons/arrowhead-right-12.svg);
   width: 12px;
   height: 12px;
   opacity : 0.6;