You'll need a more modern computer do download the zip file. You can transfer it to your PowerMac G5 using a USB thumb drive. (No browser on 10.5 in its original state can access these websites anymore, but TenFourFox can – once it's installed.)
Since this is the newest computer I have out of the several dozen that I use, the only way I can get the browser is to find someone with a newer computer to download it for me? That seems quite counterproductive. Like why make a browser that the computer it is made for cannot download it?
Or I can buy a newer computer, and then I don't need the browser at all. Seems kind of silly to me.
We used to have a workaround for this catch-22 situation when TenFourFox was still in active development, but as far as I'm aware this workaround isn't available anymore.
But really: I can see you're using Firefox 3.6 on your 'newest computer'. I.e. the web browser on that machine has not been updated in over ten years. Now you're holding us responsible that your Mac cannot connect to our secure download platform anymore? You can't be serious.