Firefox 4 requires MacOS X 10.5 or 10.6. Sorry about that. You can update to 3.6.17, but that is the end of the road for MacOS X 10.4. Actually, there's an unofficial release of Firefox code tweaked for 10.4 users with PowerPC processors. http://www.floodgap.com/software/tenfourfox/ Before playing, I recommend backing up your Firefox settings in case something goes wrong. See [https://support.mozilla.com/en-US/kb/Backing+up+your+information Backing up your information]. (You can copy your entire Firefox profile folder somewhere outside of the Mozilla folder.)