Whoops, the last version of Firefox that runs on your MacOS is the Extended Support Release of Firefox 78. (See: [[Firefox Mac OS X 10.9, 10.10 and 10.11 users move to Extended Support Release]]) You can download that from the archive server at: http://archive.mozilla.org/pub/firefox/releases/78.15.0esr/mac/en-US/ This folder has the U.S. English version. If you prefer a different language/local, click the .. link above the file name and then click into the one you want. This article has the steps to install from that .dmg file: [[How to download and install Firefox on Mac]]. Can you get reconnected with your Firefox data?