Quality Assurance (QA) is a big space, so 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 feel you have contributed to improving the next release of Thunderbird.
There are Three Areas where you can contribute to the Quality of Thunderbird. They are quickly explained below, each of them having a specific page with more details.
This activity is simple and doesn't require any special skill. You just need to not be afraid of your computer and have a bit of time to spend on the testing activities.
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 let developers know about issues with Thunderbird.
Work in this area is based on understanding the product, the protocols, and what users are seeing. No need to be too technical - but it does takes time to get acquainted with how things work, so this activity is more time consuming than testing.
More information on this activity can be found here.
- this is the #tb-qa channel on irc.mozilla.org