Firebird above, Moz below:
Code:
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\---1 \---2 \---3 \---4 \---5
So, you see, moz forks out a new branch of the development at each major release, while fb always builds from the trunk. That means that either fb or moz will always be a bit behind the development, and fb won't be forked out so that the code can be stabilised for each release they make, while moz freezes checkins just to be able to straighten out bugs.
However, most of the time the changes on the trunk doesn't affect any trunk build negatively. Generally it's the opposite, unless there's been regressions in the rendering or parsing engines.