The compatibility between Netscape, Mozilla Suite, Mozilla Firefox, et al (the gecko family of browsers), all depends on how far back you want to go.
NS 6 is based on something like Mozilla Suite 0.9.
Netscape 7.0 = Mozilla Suite 1.0.
Netscape 7.1 = Mozilla Suite 1.3.
Netscape 7.2 = Mozilla Suite 1.7.
Firefox 0.9 = Mozilla Suite 1.7
Firefox 1.0 = Mozilla Suite 1.7.5
(Upcoming Firefox 1.1 = Mozilla Suite 1.8 beta)
Then Firefox 1.0.1, 1.0.2 and 1.0.3 are minor security patches, and of course the NS 8 beta is based on both FF 1.0 and IE Trident.
AOL for Mac OS X is based on Gecko as well, but I don't know which.
I'd recommend compatibility with Mozilla 1.7, Netscape 7.2 and Firefox 1.0. These can be expected to behave identically where rendering capabilities are concerned. Versions prior to this are virtually non-existent from a statistical standpoint, and well unless you're doing something really bleeding edge whatever you do will probably work well back to at least Mozilla 1.3.
I don't have a Mac around, but in my experience things designed for gecko tend to work either perfectly or reasonably well in the KHTML browsers (Safari and Konqueror), I test in Konquorer on occasion and it usually gives me the same output as Gecko.
So really the only browser that you should have to target specifically is Explorer. As far as Opera, it's always given me mixed results. Sometimes it works and sometimes it doesn't, but I don't lose any sleep over it.
Hope that answers your questions.
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