Your useragent shows you have Mac OSX 10.5.8 when 10.6 is the minimum requirement since Firefox 17.0. https://support.mozilla.org/en-US/kb/firefox-no-longer-works-mac-os-x-10-5?

Some more info: 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/ Firefox 17 ESR has replaced the Firefox 10 ESR version, so don't update. * http://www.mozilla.org/en-US/firefox/organizations/all.html 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 to OS X 10.6.8 or later. You can see the Firefox 17 system requirements here: * http://www.mozilla.org/en-US/firefox/17.0/system-requirements/ * http://www.mozilla.org/en-US/firefox/21.0/system-requirements/ ---- * http://blog.mozilla.org/futurereleases/2012/10/04/we-bid-you-adieu-spotted-cat/