I should add that this isn't the computer I'm having problems with. I'm running win 7 SP1 (6.1.7601) and attempting to download the latest PC version of FF.

Download the full version and trash the currently installed version to do a clean install of the new version. Download a new copy of the Firefox application and save the disk image file to the desktop *Firefox 39.0: https://www.mozilla.org/en-US/firefox/all/ *Trash the current Firefox application (open the Applications folder in the Finder and drag the Firefox application to the Trash) to do a clean (re)install *Install the new version that you have downloaded *https://support.mozilla.org/kb/Installing+Firefox+on+Mac Your personal data is stored elsewhere in the Firefox profile folder, so you won't lose your bookmarks and other personal data when you uninstall and (re)install or update Firefox. *http://kb.mozillazine.org/Profile_folder_-_Firefox Create a new profile as a test to check if your current profile is causing the problem. See "Creating a profile": *https://support.mozilla.org/kb/profile-manager-create-and-remove-firefox-profiles *http://kb.mozillazine.org/Standard_diagnostic_-_Firefox#Profile_issues If the new profile works then you can transfer files from a previously used profile to the new profile, but be cautious not to copy corrupted files to avoid carrying over problems. *http://kb.mozillazine.org/Transferring_data_to_a_new_profile_-_Firefox