I cannot recommend the mentoring aspect enough. I contributed my first patch earlier this year and having contact with someone on the project to review my code

I cannot recommend the mentoring aspect enough. I contributed my first patch earlier this year and having contact with someone on the project to review my code and give me suggestions was a huge help to getting my patch accepted. It also helped me understand the code base better than if I had to make and test my own assumptions wrt design decisions and the like. I highly recommend contributing to Firefox as well. It's a very cool system.