I was just rebuilding some old machines this weekend and thought it would be expedient to just install Firefox for testing purposes rather than having Mozilla as well. They're both using pretty much the same rendering engine aren't they? In my case neither of these would be the primary browser on the machine, so am I safe eliminating Mozilla from the mix?
Firefox uses the same Gecko rendering engine as Mozilla - so no, you don't need both for testing. You might want to run the latest Firefox (which is equivalent to Mozilla 1.7) and also keep a copy of Mozilla 1.4 (the last stable release before 1.7 became the stable branch) as it was used for Netscape 7.1. There are very few changes, mostly bugfixes, but you never know.
There are even greater changes between 1.0 and 1.4, but older Mozilla variants are very rare - most users keep relatively up-to-date.
There are small differences between mozillas and netscape, those differences were more pronounced on Netscape 6.2 vs Mozilla 0.9 and Netscape 7.0 and Mozilla 1.0, the latest Netscape, 7.2 is however I believe just Mozilla gecko 1.7, as noted.