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.
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If you're using an outdated version of Firefox on Mac OS X 10.8 (Mountain Lion) or earlier, the only way you can update Firefox to a secure version is to upgrade your Mac OS to a version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.
Note: If you have Mac OS X 10.7 (Lion) or 10.6 (Snow Leopard), you will need to download OS X El Capitan before you can upgrade to the latest Mac OS.
What Is Latest Version Mac Os X
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.