There are a lot of interesting Javascript frameworks to explore. Contributing to those is challenging, so I would start with writing plugins for those. Learn how to make a simple jQuery plugin. You must have worked with some in the past. Look it up on Github. They usually have their issues listed, pick an easy one, and try to fix it. Start with small fixes. Read a lot of code. Poke around.
I do use Thunderbird, and it's working OK I guess. I just really like the UI of web Gmail. I'm not so sure if I like the idea of forwarding all of my mail places.
I'm not familiar with Rails, but in Django you are 100% tied to SQL databases. There are some efforts to make talk to NoSQL databases (like App Engine). Google actually gave up those efforts.
In Rails, I believe the database backend can be either. Look at Twitter - their using Cassandra.