Current plans for Thunderbird include to follow the 17.0 ESR (extended support) branch with releases until a new ESR branch is opened (see the summary and detailed posts on tb-planning), thus no longer moving beta versions directly to the upcoming release branch. Furthermore, only one beta is envisioned during each cycle, increasing frequency towards the Gecko 24.0 release.
It is not clear to me what the impact on SeaMonkey releases will be, so maybe someone else has more information on the discussions in the background (the IRC logs ended a year ago, not sure why). In general there appear to be two options:
- SM follows what TB did and produces releases after 2.14 against mozilla-esr17, which would ensure stability for the duration of this branch but may delay new features from entering the releases (thus, in essence the pre-rapid-release status);
- SM stays in the rapid-release scheme and builds against current mozilla-release, thus picking up improvements in the browser, but possibly introducing instabilities in the mail/news part given that there is less incentive from the Thunderbird side to fix regressions introduced by Core changes (i.e., SeaMonkey developers and other volunteer contributors may need to be increasingly active to fix those before they become relevant for the next release).