This is purely registry tweaks, no code involved. There is no custom context menu, just a single command to launch the browser.
Copy this into a reg file and install it.
WARNING use at OWN RISK. This is not a real GUID.
It probably doesn't work on other language versions, or if you installed firefox in a different location etc etc...
Windows Registry Editor Version 5.00
"InfoTip"="This Should Launch Firefox"