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.
Download OS X Lion for Mac free. Important Note: OS X 10.7 Lion has been replaced by OS X 10.8 Mountain Lion. After you install Snow Leopard you will have to download and install the Mac OS X 10.6.8 Update Combo v1.1 to update Snow Leopard to 10.6.8 and give you access to the App Store. Before upgrading check that you computer meets the minimum requirements.
If you're using an outdated version of Firefox on Mac OS X 10.8 (Mountain Lion) or earlier, the only way you can update Firefox to a secure version is to upgrade your Mac OS to a version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.
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.