Fix version drop down creation.

Retrieval of scriptUrl was not working on Firefox.

version.js

@@ -9,37 +9,41 @@ 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)
-baseUrl.pathname = baseUrl.pathname.split('/').slice(0,-1).join("/")
+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) {
-    url = new URL(url);
-    pathname = url.pathname.replace(baseUrl.pathname, "");
-    parts = pathname.split("/");
-    parts[1] = version;
-    url.pathname = baseUrl.pathname + parts.join("/");
-    return url
-}
+  function urlForVersion(url, version) {
+      url = new URL(url);
+      pathname = url.pathname.replace(baseUrl.pathname, "");
+      parts = pathname.split("/");
+      parts[1] = version;
+      url.pathname = baseUrl.pathname + parts.join("/");
+      return url
+  }
 
-function writeVersionDropdown() {
-    currentVersion = document.currentScript.parentNode.innerText;
-    document.currentScript.parentNode.classList.add("dropdown");
-    document.currentScript.parentNode.innerText = "";
-    document.write('  <span onclick="myFunction()" class="dropbtn">'+currentVersion+'</span>');
-    document.write('  <div id="myDropdown" class="dropdown-content">'); 
-    for(var version in versions) {
-        var label = versions[version];
-        if (label != version) {
-            label += "("+version+")"
-        }
-        label = "Version " + label
-        document.write('    <a href="'+urlForVersion(docUrl, version)+'">'+label+'</a>');
-    }
-    document.write('  </div>');
-};
+  function writeVersionDropdown() {
+      currentVersion = document.currentScript.parentNode.innerText;
+      document.currentScript.parentNode.classList.add("dropdown");
+      document.currentScript.parentNode.innerText = "";
+      document.write('  <span onclick="myFunction()" class="dropbtn">'+currentVersion+'</span>');
+      document.write('  <div id="myDropdown" class="dropdown-content">');
+      for(var version in versions) {
+          var label = versions[version];
+          if (label != version) {
+              label += " ("+version+")"
+          }
+          label = "Version " + label
+          document.write('    <a href="'+urlForVersion(docUrl, version)+'">'+label+'</a>');
+      }
+      document.write('  </div>');
+  };
+} else {
+  function writeVersionDropdown() {}
+}
 
 /* When the user clicks on the button,
 toggle between hiding and showing the dropdown content */