Current Firefox versions require OS X 10.6 or later, so if you want to continue using Firefox beyond Firefox 16 then you need to update to OS X 10.6.8 or later. * https://support.mozilla.org/kb/firefox-no-longer-works-mac-os-x-10-5 Firefox 16.0.2 is the last version that works on an Intel Mac with the OS X 10.5 operating system. * https://ftp.mozilla.org/pub/mozilla.org/firefox/releases/16.0.2/mac/ Another option to consider is to install the last Firefox 10.0.x ESR version as that version is more recent than the Firefox 16.0.2 version. * https://ftp.mozilla.org/pub/mozilla.org/firefox/releases/10.0.12esr/ * http://www.mozilla.org/en-US/firefox/10.0.12/system-requirements/