Fix version drop down creation.

Retrieval of scriptUrl was not working on Firefox.

version.js

@@ -9,10 +9,11 @@ const versions = {
 }
 //--- list of versions ---
 
-var scripts = document.getElementsByTagName("script"),
-scriptUrl = new URL(scripts[scripts.length-1].src);
-docUrl = new URL(document.URL);
-baseUrl = new URL(scriptUrl)
+var script = document.currentScript
+if (script && script.src) {
+  var scriptUrl = new URL(script.src);
+  var docUrl = new URL(document.URL);
+  var baseUrl = new URL(scriptUrl)
   baseUrl.pathname = baseUrl.pathname.split('/').slice(0,-1).join("/")
 
   function urlForVersion(url, version) {
@@ -40,6 +41,9 @@ function writeVersionDropdown() {
       }
       document.write('  </div>');
   };
+} else {
+  function writeVersionDropdown() {}
+}
 
 /* When the user clicks on the button,
 toggle between hiding and showing the dropdown content */