I have a signed xpi file located to *C:\Program Files(x86)\MyExtensionProject* and I created a registry value in Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Mozilla\Firefox\Extensions with the string value that name of the extension ID and value data of xpi location.
Like:
Name:
MyNewExtensionID
Data:
C:\Program Files(x86)\MyExtensionProject\myExtension.xpi
Thank you.