Starting with Firefox version 49, Firefox requires Mac OS X 10.9 (Mavericks) or higher. Visit this Apple Support page to find your Mac OS version.
If you're using an outdated version of Firefox on Mac OS X 10.8 (Mountain Lion) or earlier, the only way you can update Firefox to a secure version is to upgrade your Mac OS to a version that Apple supports. Visit this Apple Support page to learn more about upgrading your Mac.
Upgrade To Os X El Capitan
While insecure versions of Firefox will continue to work on OS X 10.8 and earlier, using an up-to-date version of Firefox on a supported version of Mac OS will provide you with the best and safest user experience.
Os X El Capitan 10.12
Go to the App Store and open the El Capitan page there. Then you must click on the ‘Download; button on the El Capitan Page. If your Mac PC is compatible to perform this upgrade, a file named ‘Install OS X El Capitan' will download to your Applications folder. The installer will open automatically after finishing the download process. After upgrading to OS X El Capitan you may experience apps that crash or that simply do not work at all but used to run properly on the previous OS before the upgrade. Bluetooth Problem. After upgrading to OS X El Capitan, you may have seen odd Bluetooth connectivity issues with the Apple Magic Trackpad. Deletion Problem.