title: telnet link does not open shell extension in firefox
body: <p>There some strange issue on my firefox 47.0.1 on Windows 7 and Windows 10. I can't open telnet links - like "telnet://some.server.org:123".</p>
  
  <p>Same links work properly on Chrome or in Firefox on Windows XP. When i click on link, Firefox suggests me to open it with Internet Shortcut Shell Extension DLL. </p>
  
  <p>I think, that the command to run is "C:\Windows\System32\rundll32.exe" "C:\Windows\System32\url.dll",TelnetProtocolHandler %l - as set in HKEY_CLASSES_ROOT\telnet\shell\open\command But nothing happens when I click "OK" or when I set "Internet Shortcut Shell Extension DLL" as default and click to a link. </p>