Storm in a teacup
Corporate (er, what do you expect, this is an IT news website!) … and I really haven't seen a G3 running for years, honestly. I found a BBC micro upstairs earlier on though :)
Your first sentence says it all. It's a tiny proportion of the user base. For a free-at-the-point-of-use software project, that is reason enough to drop it as a target platform. Having said that, I expect that if someone offered somebody enough money to cover the cost of maintaining Firefox for that platform, then it would be done.
Nobody's stopping people who want to persist with old hardware or OS releases from doing so, but they shouldn't be surprised when support in a given application for their chosen platform gradually evaporates as their proportion of the total user base of that application diminishes. I have a computer with an AGP graphics card slot, no PCI-E. So I (a) pay a bit more for a replacement card and (b) don't have access to any of the latest chipsets. That's fine, it's just progress. I can choose to spend some cash one a more modern machine, or not!
Firefox 3.6 works fine for web on all of the obsolescent platforms you cite, and will continue to do so, so where's the problem?
If it's that important to a given user to have the latest (free) application software, then either upgrade to an OS less than three years old at the time or writing (more, before this issue becomes concrete) if your computer supports it, or to a computer less than seven years old if not.
The kids will get over it (besides which, by the time this newer version of Firefox is released, they will be able to inherit a sufficiently modern machine to run it, right?)
I still don't see what all the fuss is about.
Always best to remain anonymous when conversing with people having an IQ lower than 70, by the way, well done for that ;)