fx-compat: Mostly fix About pane

Example of converting top-level XUL `<dialog/>` to
`<window><dialog/></window>`, which uses a Mozilla-provided `<dialog>`
Custom Element

chrome/content/zotero/about.xhtml

@@ -5,14 +5,14 @@
 <?xml-stylesheet href="chrome://zotero-platform/content/zotero-react-client.css" type="text/css"?>
 <!DOCTYPE window SYSTEM "chrome://zotero/locale/about.dtd">
 
-<dialog
-	id="zotero-about"
+<window
 	xmlns="http://www.mozilla.org/keymaster/gatekeeper/there.is.only.xul"
-	orient="vertical"
-	buttons="accept"
-	buttonlabelaccept="&zotero.about.close;"
-	onload="moveToAlertPosition(); sizeToContent(); document.documentElement.getButton('accept').focus();"
-	ondialogaccept="return true;">
+	onload="moveToAlertPosition(); sizeToContent(); document.getElementById('zotero-about').getButton('accept').focus();">
+<dialog
+		id="zotero-about"
+		buttons="accept"
+		buttonlabelaccept="&zotero.about.close;"
+		ondialogaccept="return true;">
 	
 	<script src="include.js"/>
 	
@@ -83,4 +83,5 @@
 			</div>
 		</vbox>
 	</vbox>
-</dialog>
+</dialog>
+</window>

chrome/content/zotero/standalone/hiddenWindow.xul

@@ -57,7 +57,7 @@
 		function openAbout() {
 			var ww = Components.classes['@mozilla.org/embedcomp/window-watcher;1']
 				.getService(Components.interfaces.nsIWindowWatcher);
-			ww.openWindow(null, 'chrome://zotero/content/about.xul', 'about', 'chrome,dialog=yes', null);
+			ww.openWindow(null, 'chrome://zotero/content/about.xhtml', 'about', 'chrome,dialog=yes', null);
 		}
 	</script>
 	<commandset id="mainCommandSet">

chrome/content/zotero/zoteroPane.js

@@ -5489,7 +5489,7 @@ var ZoteroPane = new function()
 	 * Opens the about dialog
 	 */
 	this.openAboutDialog = function() {
-		window.openDialog('chrome://zotero/content/about.xul', 'about', 'chrome,centerscreen');
+		window.openDialog('chrome://zotero/content/about.xhtml', 'about', 'chrome,centerscreen');
 	}
 	
 	/**