There are many ways you can help. Even if you have no experience, you can help make Thunderbird better. Jump in, and you too can have the satisfaction that you have contributed to improving the next release of Thunderbird.
Below are just a couple ways you can contribute to the Quality of Thunderbird. They are briefly explained, each of them having a specific page with more details.
This activity is simple, takes the least amount of time, and doesn't require any special skill.
More information here if you want to help with the testing activity.
Moderate: Maintaining the bug database
The bug database aka bugzilla is the tool used by developers to track defects in the product. The database is open to anyone who wants to help developers know about issues with Thunderbird.
Work in this area requires having some experience just using Thunderbird (a few months at least), and the ability to determine, through a user's written problem report, what the user might be seeing. You don't need to have great technical skill. It just takes a little time to get acquainted with how things operate in bugzilla.
More information on this activity can be found here.
- this is the #tb-qa channel on irc.mozilla.org