Applying a Greasemonkey script to a chrome:// Firefox extension
This is not possible, by design.
Sadly, Greasemonkey deliberately restricts itself to http:, https:, ftp:, and data: schemes by default.
You can also run on file: or unmht: schemes, or about:blank, if you explicitly set some magic options.
The developers have stubbornly refused to yield on this point, but you can easily fork the Greasemonkey code and remove the chrome: restriction if you wish.
In the source code, you would edit the file isGreasemonkeyable.js to add a line:
case "chrome":
just after the case "data": line. (Then rebuild and install your modified extension).