After download there was a circle/slash forbidding it to work in my Application folder on my Mac G5 OS 10.4.11
I went to Firefox Mozzilla website for Mac http://www.mozilla.com/en-US/products/download.html?product=firefox-5.0.1&os=osx&lang=en-US & downloaded program. I put it in Application folder even though had circle with slash and it would not work.
Opaite Mbohovái (1)
http://www.mozilla.com/en-US/firefox/5.0/system-requirements/
PPC and 10.4.11 aren't supported in Firefox 4_ versions.
For older Macs that aren't supported in Firefox 4.0 & 5.0, try TenFourFox for PowerPC's running Mac 10.4.11 & 10.5.8 .
http://www.floodgap.com/software/tenfourfox/
Or use the older 3.6.19 version.
http://www.mozilla.com/en-US/firefox/all-older.html