Hello Venetia,

The last compatible Firefox version for OSX 10.11 El Capitan is version 78.15.0esr according to this Knowledge Base article: Firefox Mac OS X 10.9, 10.10 and 10.11 users move to Extended Support Release

I found a link to download and install said version with the English/US language. You can also select other languages by clicking the ".." link next to "Dir". https://ftp.mozilla.org/pub/firefox/releases/78.15.0esr/mac/en-US/

You can then download the .dmg file and follow instructions in this Knowledge Base article to install Firefox on your Mac: How to download and install Firefox on Mac

Kindly mark this answer as the solution if it managed to help you. If it did not, kindly report back so I can help! Apologies for being unable to provide screenshots of me trying this solution out as I do not own a Mac.

Firefox 78.15.0 is the last version for your Mac OS X platform (update channel: mozilla-mac-eol-esr1) and you will no longer receive updates, so you are out of luck if you can't update your OS. Current releases require Mac OS X 10.12 or newer.

• https://support.mozilla.org/en-US/kb/firefox-mac-osx-users-esr

Firefox 78.15.0 on the Mozilla CDN server:

• https://download-installer.cdn.mozilla.net/pub/firefox/releases/78.15.0esr/