No current versions of FireFox run on PowerPC Macs any more. The last version that does was FireFox version 3.something--now FF is at v27.
However, there is an open-source version of FF ported for older Macs, TenFourFox. It is what I now use for browsing on all our PowerPC Macs. Current version is here:
HOWEVER, if you require plug-ins, the last version that supports plugins (after a modication of settings) was v17, found in this list:
Your equipment info in your profile says you are running OS 10.6.7--that OS cannot run on a PowerBook. Please check to make sure you don't have a MacBook Pro--that will change all the suggestions we have made here.