Firefox support has ended for Mac OS X 10.6, 10.7 and 10.8
Starting with Firefox version 49, Firefox requires Mac OS X 10.9 (Mavericks) or higher. If you are on an earlier version of Mac OS X, there are options for you to continue using a secure version of Firefox.
Option 1: Upgrade to a Mac OS version supported by Apple
To continue browsing safely, we recommend upgrading to a Mac OS version that Apple supports. Visit this Apple Support Page to learn more about upgrading.
While Firefox will still continue to work on unsupported versions, using Firefox on a supported version will provide you with the best and safest user experience.
Option 2: Download Firefox Extended Support Release (ESR) version 45
Firefox Extended Support Release (ESR) will continue to work with Mac OS X 10.6, 10.7 and 10.8 but will not receive updates beyond ESR version 45. Firefox ESR version 52, scheduled for release in March 2017, will no longer be supported. For more information and to download Firefox ESR, see the Firefox ESR page.