The staff at Floodgap Software have worked overtime to keep us older Apple iMac people happy by reworking the Mozilla source code for Firefox. This allows them to produce a version of this famous browser for the PowerPC chipsets found in older Apple iMac, Powerbooks, etc. Supported chipsets include G3, G4 and G5 processors. You can look at their Floodgap Intro Page here: http://www.floodgap.com/software/tenfourfox/
Grab a download of TenFourFox Version 10.0 beta 1 (Firefox) for your iMac PowerPC Chipset system here: http://code.google.com/p/tenfourfox/downloads/list. You will need to know which processor your system has.
The release notes can be found here : http://code.google.com/p/tenfourfox/wiki/ReleaseNotes10Beta1
Developer notes can be found here: http://tenfourfox.blogspot.com/2012/01/100b1-stable-available.html