/moved thread from Firefox for iOS (for iPhone/iPad) to Firefox section as you have macOS.

The old Firefox 78.15.0esr that you appear to have is the most current you can run on the old macOS 10.9, 10.10,10.11 https://support.mozilla.org/en-US/kb/firefox-mac-osx-users-esr

Firefox 79.0 to Firefox 115 ESR requires macOS 10.12, 10.13, 10.14 at minimum to run while Firefox 116.0 or later requires macOS 10.15 or later to run. https://support.mozilla.org/en-US/kb/firefox-users-macos-1012-1013-1014-moving-to-extended-support

If you can upgrade your macOS to at least 10.12 then you can use the still supported with updates Firefox 115 ESR. It is currently at 115.18.0esr and will be updated till at least 115.21.0esr in March 2025 unless Mozilla extends updates again.

If you do manage to upgrade to macOS 10.12+ then you can download the Firefox 115.18.0esr.dmg from below as each folder is the language. https://archive.mozilla.org/pub/firefox/releases/115.18.0esr/mac/ (will not work on macOS 10.11.6)

Other web browsers have already dropped support for macOS 10.9 to 10.14

You may be able to run a newer version of MacOS, but this is not considered a regular update to Mavericks, so the update checkers might not inform you. Apple calls it an "upgrade."

According to an old Apple support thread, you can go to the version's page on the store and there is a compatibility checker feature.

• Monterey (12): https://apps.apple.com/us/app/macos-monterey/id1576738294?mt=12
• Big Sur (11): https://apps.apple.com/us/app/macos-big-sur/id1526878132?mt=12
• Catalina (10.15): https://apps.apple.com/us/app/macos-catalina/id1466841314?mt=12
• Apple support article: How to download and install macOS

I saw a suggestion to try it in Safari if Firefox and Chrome don't allow the site to fully determine compatibility.

As a Windows person, this is all second-hand.