Just FYI, Hacktoberfest is there again!
You can create 4 pull requests from October 1st to October 31st and earn a T-Shirt.
As for my case, i.e. my add-ons and libs, I’ve also tagged around 30 issues that are likely well-suited for this project.
I guess this can be combined quite nicely with Mozilla’s add-on workshop here, at least if beginners don’t know extensions very well yet: