Hopefully when you replaced the old Firefox app the profile folder (where Firefox data is stored) wasn't affected. As mentioned in [[Firefox Mac OS X 10.9, 10.10 and 10.11 users move to Extended Support Release]], you need the Extended Support Release of Firefox 78. The last in the series for U.S. English can be downloaded here: http://archive.mozilla.org/pub/firefox/releases/78.15.0esr/mac/en-US/ If you prefer a different language or locale, click the .. link on that page and then click into the correct one. (Usually it's the one this site inserts into the URL as you browse.) Please check the following article for what to do with the .dmg file after you download it: [[How to download and install Firefox on Mac]]. Can you get back up and running?