installing older versions of firefox

Your Macbook is using '''macOS''' as iOS is the mobile OS used on iPhone's and earlier iPad's as more current iPad's uses iPadOS. The latest update for macOS Mojave is 10.14.6 (ten.fourteen.six) So Firefox 115 ESR is the latest you can run on macOS 10.14 as Fx 116.0 and later requires macOS 10.15 (ten.fifteen) or later to run. The current version is 115.14.0esr and 115.15.0esr in early September will be the last update unless Mozilla decides to extend support. https://support.mozilla.org/en-US/kb/firefox-users-macos-1012-1013-1014-moving-to-extended-support You can download Firefox 115.14.0esr for macOS at https://www.mozilla.org/firefox/all/#product-desktop-esr However if you really need the ancient and rather vulnerable Firefox 31.0 released ten years ago on July 22, 2014 then see https://archive.mozilla.org/pub/firefox/releases/31.0/mac/ /moved thread from Firefox for iOS to Firefox section.