You may use this tool to create a Firefox extension (.xpi) from a greasemonkey script.
Type in the appropriate details below, you may leave the default random GUID if you do not have one, or replace it with your own value, which you should definitely do for upgrades to an existing extension.
When pasting in the script, include the ==UserScript== block, as the compiler reads data from there (name, description, includes/excludes).