Working on Firefox can be daunting when you first start as a developer.
There is an amazing amount of Mozilla specific technology you need to learn; in addition you may not have had the opportunity to work with things like mercurial patch queues (MQ).
On the other hand: There is a massive collection of documentation that you have to keep looking up.
When I started as a platform developer at Mozilla 3 months ago, I started making a cheat sheet of common information I would have to frequently look up. I will continue to update this page as I continue to learn every day at Mozilla.
I highly encourage anyone looking to contribute to an open source project to look into contributing to Mozilla.
Not only will you be helping an open source non-profit organization, but you will also connect with extremely smart peers, have extra resume flair, and learn a ton. The experience of contributing (or working) for Mozilla will forever change you as a developer.