encyclo - 1:33 am on Jul 11, 2004 (gmt 0)

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.