Thunderbird 3.1.20 (three.one.twenty) Mac OS X 10.4.x and later Macintosh computer with an Intel x86 or PowerPC G3, G4, or G5 processor https://archive.mozilla.org/pub/thunderbird/releases/3.1.20/mac/ and pick your language. Thunderbird 5.0 then required Mac OS X '''10.5''' and '''Intel''' cpu as PPC was dropped. http://www.mozilla.org/thunderbird/5.0/system-requirements/ https://www.mozilla.org/thunderbird/releases/ For Firefox https://www.mozilla.org/firefox/releases/ Firefox 4.0 the PPC support was dropped and required a Intel cpu and OSX 10.5 at minimum up until Firefox 16.0.2. Firefox 17.0 to current has required OSX 10.6+ Firefox '''3.6.28''' was the last to run on the old Mac OS X 10.4.x and PPC. Since you have a PPC Mac there is this third-party option for Firefox which you appear to be aware of based on useragent. http://www.floodgap.com/software/tenfourfox/ https://support.mozilla.org/en-US/kb/firefox-no-longer-works-mac-os-10-4-or-powerpc