I want to switch gears and talk about my own experience in starting to contribute to an open source project. First I want to make a big disclaimer. Applying to OPW and choosing from active projects with vetted mentors made things A LOT easier. An advantages of this is that you have someone to contact who is first and foremost interested in mentoring someone, and willing to do so for a substantial amount of time.
I hope, however, that for someone who is reading this who is either not eligible to apply or not obsessed with OPW like I am, finds that my experience is generally translatable. Had I not found OPW, I would most likely either be at the Mozilla or GNOME bugzilla page facing similar dilemmas and wondering where to go next. It probably would have been more of a hit and miss, but I think I would of followed a similar path as above.