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/ You can consider to install the latest Firefox 10.0.x ESR version that will still receives updates until it is replaced by Firefox 17 ESR (possibly in February 2013). Firefox 17+ versions require OS X 10.6 or later, so if you want to continue using Firefox beyond Firefox 16 then you need to update your OS. You can see the Firefox 18 system requirements here: *http://www.mozilla.org/en-US/firefox/18.0/system-requirements/ *http://blog.mozilla.org/futurereleases/2012/10/04/we-bid-you-adieu-spotted-cat/