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I think it is a pun on NCSA Mosaic, on which the founders of Netscape had worked before starting Netscape, and Godzilla, which also explains the Mozilla peoples preference for dinosaur-like monsters.
Netscape 1-4 always identified itself as Mozilla/1 - Mozilla/4.
The current Mozilla releases identify themselves as Mozilla/5, because they were originally based on the code for the never released Netscape Navigator 5.
This also explains why Netscape released a Netscape 6 and 7, but never a 5. It formed the basis for Mozilla, but was ditched early on.
René.