Hi gworgan, This is a known issue caused by a corrupted installation after a failed update. The workaround is to uninstall and then re-install Firefox to fix this issue while our team is working on a more permanent fix. You shouldn't lose any data if you download the full installer from [https://www.mozilla.org/firefox/all/#product-desktop-release our official source]. So sorry for taking a long time to get back here with an actual resolution.