"Specifically the Mortgage Lifter that is the result of cross breeding four different varities. Why will the Mortgage Lifter, a Heirloom, produce the same fruit from seed, when an Early Girl, a Hybrid, will not?"

Jan's explanation is accurate. Mortgage Lifter was repeatedly selected for several generations to stabilize it after the initial hybrids of hybrids. It may not be 100% stable today. But you could continue the selection process to develop your own strain of Mortgage Lifter.

Early Girl is an F1 hybrid. That's not to say that you couldn't save seed from Early Girl and get some interesting F2 variations, some of which you might like. The interesting thing about growing F2s is that you can see new characteristics that were different from either F1 parent. If you found an F2 that you liked, or several, you could save seed from it (them) and the resulting F3s would still vary a lot, but each group would be somewhat centered around the characteristics of the F2s that you selected.

In several generations of such repeated selection you could stabilize one or more of your own varieties from the F1 Early Girl.

Then you could go to the next level and cross a couple of those varieties to produce a new F1 hybrid tomato. By continually improving your inbred strains you can also improve the F1 hybrids that you can produce from them. There are a lot of ways you could combine, segregate, and recombine tomato genes.

-- Burton --