Firefox since Fx 79.0 and later including the current Fx 97 and Fx 91 ESR requires Mac OS X 10.12 (ten.twelve) at minimum.

Besides updating your OS to Mac OS X 10.12 (ten.twelve) or later, the most current Firefox version you can install on mac OS 10.11.6 is Fx 78.15.0 ESR though it is EOL with no more updates.

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

https://www.mozilla.org/firefox/78.0/system-requirements/ https://www.mozilla.org/firefox/79.0/system-requirements/ https://www.mozilla.org/firefox/97.0/system-requirements/

In case you ever need the Firefox 78.15.0esr file. https://archive.mozilla.org/pub/firefox/releases/78.15.0esr/mac/en-US/ other locales at https://archive.mozilla.org/pub/firefox/releases/78.15.0esr/mac/

Thank you for sharing the Firefox browser version I need Fx 78.15.0

Where do I find this version to download to my computer?

As James sent (as the last two links), you can get the US English version at https://archive.mozilla.org/pub/firefox/releases/78.15.0esr/mac/en-US/

For other languages you can get, look https://archive.mozilla.org/pub/firefox/releases/78.15.0esr/mac/

Note that you will not get security updates.

Thank you for sharing the Firefox browser version I need Fx 78.15.0 Where do I find this version to download to my computer?

It was in my post as you just download the Firefox 78.15.0esr.dmg and install it.

In case you ever need the Firefox 78.15.0esr file. https://archive.mozilla.org/pub/firefox/releases/78.15.0esr/mac/en-US/ other locales at https://archive.mozilla.org/pub/firefox/releases/78.15.0esr/mac/