Select and uncheck Use a background service to install updates . Important Considerations
Once the .dmg file finishes downloading, complete the local setup using the standard macOS installation process:
Using outdated browsers on old operating systems like 10.10.5 (Yosemite) exposes you to significant security risks, including malware, phishing attacks, and the inability to load modern encrypted websites (HTTPS).
78.15.0esr/mac/en-US/
If you previously used a newer version of Firefox on a different OS, you may be prompted to create a new profile because version 78 cannot read data from newer versions (like Firefox 130+). Critical Security & Compatibility Notes
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If your hardware allows it, experts recommend upgrading your macOS to at least 10.15 Catalina
However, the latest versions of Firefox no longer support OS X 10.10.5. Mozilla ended support for 10.10, 10.11, and 10.12 after Firefox 78 (ESR) and Firefox 79+.
This guide provides the exact steps to locate, download, and safely install the last compatible Firefox update for OS X Yosemite, along with crucial safety tips for browsing on legacy software. Why the Standard Firefox Download Won't Work