For older Macs that aren't supported in Firefox 4.0, try TenFourFox for PowerPC's running Mac 10.4.11 & 10.5.8 .
http://www.floodgap.com/software/tenfourfox/

The failure to post system requirements is inexcusable. It cost me a lot of wasted time and effort, and in the end, I had to re-download and re-install version 3.6.16. The entire problem could have been avoided if you had just posted the system requirements in the first place. This experience has undermined my trust in the entire Mozilla operation.

Thank you for posting that link the-edmiester. I have already re-downloaded firefox 3.6.16 but will give tenfourfox a try. However, not to point the finger at you edmiester, my original question has still not been answered. Is it too much to ask for system requirements when I am prompted to download a new version of Firefox? I realize my computer is out of date and the longer I have it the less I'll be able to use it in this brave new internet world and I don't expect Mozilla to continually pander to me and others like me by keeping there web browsers compatible with older machines but I am fine with using older versions of Firefox. 3.6 works fine and when I saw that there was an update I thought "cool a newer version that would logically work better and faster than the one I was using" but that was clearly not the case. Like Redbeard I was forced to find an older version, re-download and re-install. Luckily I still had Safari, I shutter to think what would have happened if I had deleted it. As well, like Redbeard, this totally undermines my confidence in Firefox and I would hope that Mozilla cares enough about their users, you know the ones that create the need for their jobs, to at least address things like this in a timely respectful fashion.

I not only can't use 4 (I'm also 10.4.11) but now it won't let me redownload 3.6.16. Guess I'm stuck with safari. pffft.