You need an older ESR for Mac OS X 10.11 Mac users on OS X 10.12/10.13/10.14 been moved to the 115 ESR update channel since your macOS X 10.14 platform no longer is supported (distributionID: mozilla-mac-eol-esr115), current releases require macOS 10.15 as a minimum. *https://support.mozilla.org/en-US/kb/firefox-users-macos-1012-1013-1014-moving-to-extended-support Firefox 78.15.0 ESR is the last version for Mac OS X 10.9/10.10/10.11 (distributionID: mozilla-mac-eol-esr1) and you will no longer receive updates. *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/

Thank you for the quick reply. I downloaded Mac OS 10.13.6 (High Sierra) from Apple's app store (it is free) and installed it on the old Macbook that had been running OS 10.11.6. Then I was able to run the regular Firefox ESR.