How to Download Firefox for Mac OS X 10.10.5 (Yosemite) If you’re still using a classic Mac running OS X 10.10.5 Yosemite

Alternative Browsers: If Firefox 78 does not meet your needs, some users on legacy macOS versions look toward community-maintained browsers like Pale Moon or SeaMonkey, though compatibility varies. 

1. Turn on “Always use HTTPS”

Old OS versions are vulnerable to network attacks. Go to Settings > Privacy & Security and scroll to HTTPS-Only Mode. Enable “Enable HTTPS-Only Mode in all windows.”

Mozilla moved users on OS X 10.9, 10.10, and 10.11 to the Extended Support Release (ESR) channel to provide a few extra years of security. However, maintaining support for decade-old operating systems became unsustainable as web technologies evolved. Mozilla Support Better Alternatives for Your Mac

: Firefox 78.15.0esr is the terminal build for macOS versions 10.9 through 10.11. Security Status

3. User Interface

If you attempt to install a newer Firefox (version 80 or above) on 10.10.5, the installer will fail. You need the legacy build.

Alternative: Using the Firefox Updater

Recommendation: While this version allows you to browse the web, it is no longer receiving security patches. For better security, consider upgrading your macOS if your hardware supports it (e.g., to macOS 10.15 Catalina). Where to Download