import and export based on http://add0n.com/useragent-switcher.html#IDComment1079854533

extension/data/popup/index.html

@@ -78,7 +78,7 @@
           <td>userAgent</td>
           <td colspan="3">
             <div hbox>
-              <input id="ua" type="text" placeholder="Your preferred user-agent string" title="To set blank user-agent string, use 'empty' keyword">
+              <input id="ua" type="text" placeholder="Your preferred user-agent string" title="To set a blank user-agent string, use the 'empty' keyword. To construct a custom user-agent string based on the current browser's navigator object, use ${} notation. Whatever is inside this notation is read from the 'navigator' object. For instance, to append a string to the default user-agent, use '${userAgent} THIS IS THE APPENDED STRING'">
             </div>
           </td>
         </tr>