Yup, all of them in fact, it's a very tangible proof you can work on the subject. Sadly, I do mostly Python and JS so I can't mentor you but as mentionned in ot

Yup, all of them in fact, it's a very tangible proof you can work on the subject. Sadly, I do mostly Python and JS so I can't mentor you but as mentionned in other comments, lot of projects are looking for contributors, Firefox is good project for example.