Yes, it is open source! (See details here: https://addons.mozilla.org/en-US/firefox/addon/wingman-jr-filter/license/) I do not have the source in a standalone repository (it is in a repo with other source I do not wish to open at this time), but you can download the XPI file by going to the addon to enable it and right-clicking and "Save Link As". The XPI file can simply be changed to a .zip extension and opened up to see the source.
Let me know if you have any questions; hopefully the source is short and easy to follow.

(UPDATE: I now have a GitHub here - https://github.com/wingman-jr-addon/wingman_jr)