There is a simple way to do it.
In the Run menu, you can see options like "Lunch in firefox" and more.
These options are written in a file called shortcuts.xml, located in the folder where you installed NP++.
Open this file, and inside <UserDefinedCom mands> you'll find this row:
Code:
 <Command name="launch in Firefox" Ctrl="yes" Alt="yes" Shift="yes" Key="88">firefox
...