Firefox 4 versions and later do not run on a PowerPC Mac. Firefox 4 and later require at least OS X

Firefox 4 versions and later do not run on a PowerPC Mac.

Firefox 4 and later require at least OS X 10.5 and an Intel Mac.

http://www.mozilla.com/firefox/5.0/system-requirements/

For an unofficial Firefox 4 compatible version (TenFourFox) that runs on PowerPC Macs with OS X 10.4.11 or OS X 10.5.8 you can look at:

http://www.floodgap.com/software/tenfourfox/

Firefox 3.6.x (Mac OS X 10.4 and later) can be found here:

http://www.mozilla.com/en-US/firefox/all-older.html