hello sergio53, mozilla has ceased support for the powerpc platform a while ago, but you could look at tenfourfox.com, which provides current builds of the browser based on mozilla code.

The last Firefox version from Mozilla that worked on PPC was the old 3.6.28 as Firefox 4.0 to current has required Intel hardware.

You are better off using say TenFourFox as mentioned above.

Do note that it does not use Plugins since the Plugins for PPC are rather old and insecure now. Where they mention Add-ons they really meant Extensions as Add-ons is a word that groups Extensions, Plugins, Themes and such. https://code.google.com/p/tenfourfox/wiki/PluginsNoLongerSupported