Since you have a PowerPC and not Intel hardware you can use the third-party TenFourFox. http://www.floodgap.com/software/tenfourfox/ however Plugins are not supported due to be old/vulnerable. Though TenFourFox does support Extensions unless they are say OS specific or for a more current version.

Firefox 16.0.2 was the last to work on the old Mac OSX 10.5.x as Firefox 17.0 to current has required OSX 10.6+ since https://www.mozilla.org/en-US/firefox/16.0/system-requirements/ https://www.mozilla.org/en-US/firefox/17.0/system-requirements/

Hello

James said

Firefox 16.0.2 was the last to work on the old Mac OSX 10.5.x as Firefox 17.0 to current has required OSX 10.6+ since https://www.mozilla.org/en-US/firefox/16.0/system-requirements/ https://www.mozilla.org/en-US/firefox/17.0/system-requirements/

Firefox 16.0.2 was the last to work on the old Mac OSX 10.5.x with Intel processor, with PPC the latest firefox for Mac OSX 10.5.x is 3.6.28 :-)

thanks