Firefox release notes.
Firefox ESR current release notes.
Thunderbird current release notes.
SeaMonkey current release notes.
Upcoming Firefox release dates are listed at whattrainisitnow.com.
Thunderbird approximately (i.e. not strictly) follows the ESR schedule listed there.
The full list of Firefox release notes is available on mozilla.org.
The full list of Thunderbird release notes is available on mozilla.org. Old releases can be found at releases directory.
Archive of 2001–2015 releases by date.