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/

And from Mozilla the newest for PPC Mac is Fx 3.6.19 found at http://www.mozilla.com/en-US/firefox/all-older.html

As of Firefox 4.0 and newer it is Mac OSX 10.5.+ and Intel that is required.

try TENFOURFOX.

tenfourfox site is not working today, Aug 23, 2011 and says it might be back in action within a week. /firefox/all-older.html has version 3.6.20 which will not run on my powerpc Mac.

This is the blog by the TenFourFox developer: http://tenfourfox.blogspot.com/2011/08/60r-available.html

READ VERY CAREFULLY. A release candidate version 6, plugins disabled, etc.

Links are at the bottom of his 'Tuesday, August 16, 2011 blog based on your system architecture: G3, G4/7400, G4/7450, G5