Hi Cliff, is this correct regarding your CPU:

Macintosh; PPC Mac OS X 10_5_8

Firefox has been limited to Intel processors for a long time now. There is a "fork" of the Firefox source code that a third party has adapted to run on the G3, G4 and G5:

http://www.floodgap.com/software/tenfourfox/

If you actually have an Intel processor, it's not good at all. Firefox 16 was the last which had 10.5 in its System Requirements: https://www.mozilla.org/firefox/16.0/system-requirements/

If a PowerPC then the rather old Firefox 3.6.28 was that last to still run on PPC. https://support.mozilla.org/en-US/kb/firefox-no-longer-works-mac-os-10-4-or-powerpc

If it is a Intel Mac then Firefox 16.0.2 was the last to run on OSX 10.5.x https://support.mozilla.org/en-US/kb/firefox-no-longer-works-mac-os-x-10-5

As said TenFourFox is your only real option of it is a PowerPC Mac.