Firefox 17.0 to 47.0 required macOS 10.6 at minimum to run as Fx 48.0 then required macOS 10.9 at minimum. So Firefox 16.0.2 was the last to still run on the ancient EOL macOS 10.5, that is if it was a Intel system and not a PowerPC. https://www.mozilla.org/en-US/firefox/16.0/system-requirements/ https://www.mozilla.org/en-US/firefox/17.0/system-requirements/ https://www.mozilla.org/en-US/firefox/47.0/system-requirements/ https://www.mozilla.org/en-US/firefox/48.0/system-requirements/ https://archive.mozilla.org/pub/firefox/releases/16.0.2/mac/ is for Intel. However since you have a PowerPC the TenFourFox builds were the only option for a time as I forget when Mozilla dropped PPC. . https://sourceforge.net/projects/tenfourfox/ https://www.floodgap.com/software/tenfourfox/