I don't know a lot about either variety, so I am taking it that you have your facts correct.
It can be readily accepted that an F1 hybrid plant (the immediate cross between 2 selected varieties) will be heterozygous (will carry pairs of unlike chromosomes) and will give you a variety of different seedlings.
Species plants are homozygous - both chromosomes of a pair being identical. Varieties also approach homozygosity because of diligent selection (or, in the case of naturally formed varieties - through isolation).
If the Mortgage lifter is the result of cross breeding 4 different varieties, then it will give mixed offspring UNLESS it is also the product of a selection program. When a desired type is selected over a number of generations, it again approaches a homozygous state, and will breed relatively true. There will not be absolute homozygosity, so some variation may creep in occasionally, and this can be again selected for.
This is how most selected strains of garden annuals work. You get some variation of course, but it is minor compared to the variation you get when you cross an F1 hybrid.
Cheers, jan

BigCarl,

"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 --