You're not completely there, either.
10.5 works on some PPC systems, but not all as it needs 867Mhz of processor or better. Yes, it is possible to hack the installer to install anyway..
If you're running a G3 then it's probably too slow to view modern websites, but that doesn't apply to a half decent G4. You've got a system that's fully capable of running a modern web browser, but Apple deigns it insufficiently powerful to run 10.5.
It comes down to security fixes. Apple appears to have stopped fixing 10.4 (there's no actual statement that I can see, as opposed to MS' crystal clear lifecycle policies) so perhaps Mozilla feel as if they shouldn't bother either. The real question is not necessarily the next browser - clearly 3.7 is a nice to have, but Windows 2000 (still a supported product) is still stuck on an (up to date, security patched) IE 6..
I personally think Mac users should petition Apple for dropping support for a product that's only five years old, as opposed to the 10 and 13 year support of Vista and XP respectively. If OS X is supposed to be comparable to Windows, it should be held to similar support standards. Ditto commercial Unix. Once that's achieved (good luck) Mozilla should be given some gentle encouragement too..
I suppose I should double check Mozilla's security fix roadmap, but for the moment that's left as an exercise for the reader.
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