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If you're using an outdated version of Firefox on Mac OS X 10.8 (Mountain Lion) or earlier, the only way you can update Firefox to a secure version is to upgrade your Mac OS to a version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.
Latest Os X Version Mac
OS X Mavericks is quite good, and is at least optimized for most hardware. On a brand new Mac, OS X Yosemite runs quite well, however. Fortunately, most 2010 Macs came with physical reinstall DVD volumes of Mac OS X Snow Leopard, so you should be good to go with that. OS X Mountain Lion (version 10.8) is the ninth major release of macOS, Apple Inc.' S desktop and server operating system for Macintosh computers. OS X Mountain Lion was released on July 25, 2012 for purchase and download through Apple's Mac App Store, as part of a switch to releasing OS X versions online and every year, rather than every two years or so.
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.
Update Os X 10.11 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.