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.
Mac OS X Lion (version 10.7) is the eighth major release of macOS, Apple's desktop and server operating system for Macintosh computers. A preview of Mac OS X 10.7 Lion was publicly shown at the 'Back to the Mac' Apple Special Event on October 20, 2010. The OS X Lion Update is the final update to Mac’s OS X Lion operating system. As the last release for the OS, it’s recommended by Apple that users update their system to ensure the safest and fastest use possible. OS X El Capitan (10.11) OS X Yosemite (10.10) OS X Mavericks (10.9) OS X Mountain Lion (10.8) OS X Lion (10.7) Mac OS X Snow Leopard (10.6) Mac OS X Leopard (10.5) Supported Configurations Include: FileVault 1 / FileVault 2. Core Storage (Fusion Drive). Flash / SSD drives. Internal / External Volumes. Time Machine Volumes. RAID (Promise, G-RAID.
Mac OS X Leopard (version 10.5) is the sixth major release of macOS, Apple's desktop and server operating system for Macintosh computers. Leopard was released on October 26, 2007 as the successor of Mac OS X 10.4 Tiger, and is available in two editions: a desktop version suitable for personal computers, and a server version, Mac OS X Server. Download Mac OS X El Capitan now from Softonic: 100% safe and virus free. More than 4229 downloads this month. Download Mac OS X El Capitan latest ver.
If you're using an outdated version of Firefox on Mac OS X 10.8 (Mountain Lion) or earlier, the only way you can update Firefox to a secure version is to upgrade your Mac OS to a version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.
Mac Os X 10.9 Update
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.