Do not use the standard "Download" button on the main Firefox site, as it will likely serve a version that requires macOS 10.15 or later. Instead, visit the Mozilla FTP Archive and download the .dmg file for version 78.15.0esr .
Open it from your Applications folder. You may see a security warning stating the app was downloaded from the internet; click Open to proceed. Key Considerations and Limitations Install firefox for Mac IOS 10.11.6 - Mozilla Support Firefox Mac Os 10.11.6
For a test run without replacing anything: Do not use the standard "Download" button on
Let’s be blunt:
Mac OS X El Capitan (version 10.11.6) was released in 2015 and was the final version of OS X before Apple rebranded to "macOS" with Sierra. It is widely remembered as a stable, refined operating system that focused on performance and reliability. You may see a security warning stating the
| Feature | Safari 11 (pre-installed) | Firefox 78 ESR | | :--- | :--- | :--- | | | No | Yes | | Modern JavaScript (ES2021) | Fails on many sites | Partial support | | YouTube (H.264/VP9) | Stuttering, errors | Smooth playback | | Password Manager | iCloud Keychain (broken old version) | Firefox Lockwise (works) | | Extensions | Virtually zero | uBlock Origin, Dark Reader, etc. | | Security Updates | None since 2018 | Community backports until 2025 |
If you need better security or modern features: