After updating from 17.0.1 to 18.0.1 Firefox will not make any external connections in Windows XP (SP3 running all updates as of the date of this post. ) Preceeding the failure during the installation of 18.0.1 the update process the installation message box states "Checking compatibility of add-ons" and apparently hangs. After the firefox process is killed. Firefox is restarted and will not connect to any website, although internal cached links work - initial firefox start page and setup page "about:config" Reverting to the previous 17.0.1 resolves the issue.