Here ya go. https://support.mozilla.org/en-US/kb/firefox-mac-osx-users-esr#firefox:mac:fx91&firefox-enterprise:linux:

I think actually you might need the previous one (78 instead of 91): https://archive.mozilla.org/pub/firefox/releases/78.15.0esr/mac/en-US/ If you want a language-locale other than U.S. English, click the .. link on that page and then click into the desired locale. Once you have the .dmg file, the installation steps are the same as this support article: [[How to download and install Firefox on Mac]].