Thunderbird/Security And Stability Releases
From MozillaWiki
Security and Stability releases are minor updates to our major releases. As the name suggests they are designed to address security and stability issues found since the release.
They typically happen every 6 to 8 weeks, however, if a security exploit or other serious issue is found, that may cause a release to happen outside of the regular cycle.
The next release schedule is typically listed on the Releases page.
Developer rules for the branches are found here
