As per the topic title, looking for useful resources and general advice before starting add-on development proper! Thanks!
The official resource to learn about add-on development is https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/What_are_WebExtensions
As the very start notes, you’ll want to know web technologies like JS, HTML and CSS. To learn these, you could start at https://developer.mozilla.org/en-US/docs/Learn
Also check out this nice overview site: