Sorry about that, support ended for 10.11.x some time ago ([[Firefox Mac OS X 10.9, 10.10 and 10.11 users move to Extended Support Release]]). You can download the last Extended Support Release of Firefox 78 from the following server. This address is for U.S. English. if you prefer a different language/locale, click the .. button at the top of the file list to return to the page with all the locale links, then click into the one you want. https://archive.mozilla.org/pub/firefox/releases/78.15.0esr/mac/en-US/ For more general reference, see: [[How to download and install Firefox on Mac]]. Please be careful when trashing any unwanted version NOT to remove your data.