- MailNews - For all MailNews code that overlaps between SeaMonkey and Thunderbird.
- SeaMonkey - "A community effort to develop the SeaMonkey all-in-one internet application suite."
- Calendar - Calendaring add-on for Thunderbird known as Lightning, stand-alone product Sunbird.
- Raindrop - An exploration in messaging innovation
- Correo - Correo "blends technology from two popular Mozilla projects, Camino and Thunderbird, to create a polished native Macintosh application."
- Penelope - Penelope is an open source extension to Thunderbird based on Qualcomm's Eudora mail client.
- Spicebird - A collaboration suite based on Thunderbird (discontinued).