Fun fact, while I lead the #PaleMoon Add-ons Team, we also manage the add-ons for #Basilisk and #Interlink too. Much like those applications share a common platform (#UXP), the Add-ons Site is similarly built.
https://addons.palemoon.org
https://addons.basilisk-browser.org
https://interlink-addons.binaryoutcast.com/extensions
@ryan659 Thanks a lot!
@saf Currently the documentation for that is a bit lacking (as it was even when Firefox used it) but you can use MDN for general XUL information, and a tutorial to write a basic "hello world":
https://developer.mozilla.org/en-US/docs/Archive/Mozilla/XUL
https://www.xul.fr/tutorial/first-program.html
You can also inspect existing add-ons (XPI files are just .zip). I'd really recommend posting on the Pale Moon forums if you have any specific questions though!
https://forum.palemoon.org