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If you're using an outdated version of Firefox on Mac OS X 10.8 (Mountain Lion) or earlier, the only way you can update Firefox to a secure version is to upgrade your Mac OS to a version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.
Apple - Support - Downloads
As an operating system, Mac OS has always tried to make things sleek and minimalist on your desktop. However, in the older versions, the sync between Mac and iPhone led to a lot of clutter. To counter that, Apple added Desktop Stacks to make your desktop organized according to the criteria you desire. In essence, High Sierra took what was good with Sierra and developed it into a more comprehensive software. High Sierra emphasizes the already-existing benefits of MacOS previous system, Sierra. There are few bells and whistles, but the changes are noticeable and positive. The Apple file system is the most significant feature it brought.
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.
How To Download Firefox For Mac OS El Capitan & Sierra
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.