Because modern versions of Firefox require (10.12) or later, the official download page will often serve a version that is incompatible with your system. You should download the specific legacy installer from the Mozilla Archive . Installation Guide for El Capitan
To download for OS X El Capitan (version 10.11), you must use Firefox version 78.15.0esr . This is the final version supported for this operating system. Key Download Information
: A window will appear with the Firefox icon. Drag this icon into the Applications folder.
: Double-click the downloaded Firefox 78.15.0esr.dmg file in your Downloads folder.
: Open your Applications folder and double-click Firefox. You may see a security prompt; click Open to confirm. Important Compatibility & Security Notes
: Visit the Mozilla FTP server and download the file ending in .dmg .