Security and compatibility tips
+--------------------------+-----------------------------------------------------------+ | Challenge Area | Impact on OS X 10.10.5 Users | +--------------------------+-----------------------------------------------------------+ | Security Vulnerabilities | No security patches since late 2021; open to modern exploits. | | Broken Layouts | Modern web frameworks (CSS/JS) may fail to render properly. | | SSL / TLS Errors | Outdated root certificates can cause "Your connection is not private" errors.| | Extension Support | New extensions on the Mozilla Add-ons marketplace will not install. | +--------------------------+-----------------------------------------------------------+ Alternative Solutions for Older Mac Hardware
To perform a clean uninstall, quit Firefox, then drag the Firefox icon from your "Applications" folder to the Trash. Also, delete the Firefox folder in ~/Library/Application Support/ to remove your profiles and user data.
On this page, you can manually select your preferred language and operating system version. This is particularly useful if you are setting up Firefox on multiple machines or need a non-default language package.
78.15.0esr/mac/en-US/
in about:config → set to 10 (reduces RAM use).
After successfully completing the , you need to configure Firefox to work safely on an outdated OS.
Ensure that the browser does not attempt to auto-update to a modern version, as this will break the application on your system. Navigate to Firefox Preferences > General > Firefox Updates and select "Check for updates but let you choose to install them." Long-Term Alternatives for OS X 10.10.5
A: Mozilla has not announced any immediate plans to drop support for macOS 10.15. Since Firefox 116.0, Catalina has been the minimum requirement, and it remains fully supported. However, as macOS evolves, support for older operating systems may eventually be phased out. For now, Catalina users can continue using the latest versions of Firefox without concern.
May 2026 Reading Time: 6 minutes
Mainstream Firefox (Versions 78+) dropped support for OS X 10.10 and 10.11 in mid-2021. The last compatible series is Firefox 78 ESR (Extended Support Release). However, a later ESR – Firefox 91 ESR – initially required macOS 10.12+. But community reports indicate Firefox 78.15.0esr is the final official build for 10.10.5.
If your hardware allows it, experts recommend upgrading your macOS to at least 10.15 Catalina
