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If you're using an outdated version of Firefox on Mac OS X 10.8 (Mountain Lion) or earlier, the only way you can update Firefox to a secure version is to upgrade your Mac OS to a version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.
Mac Os X Lion 10.7.5 To El Capitan
Os X El Capitan Dmg
Open that app from your Applications folder to begin installing the operating system. MacOS Sierra 10.12 can upgrade El Capitan, Yosemite, Mavericks, Mountain Lion, or Lion; OS X El Capitan 10.11 can upgrade Yosemite, Mavericks, Mountain Lion, Lion, or Snow Leopard; OS X Yosemite 10.10can upgrade Mavericks, Mountain Lion, Lion, or Snow Leopard. Hello, ive created this theme for Mac OS X Lion, only tested in 10.7.3 and 10.7.5, but u shall try in any version of Lion, it has the same traffic light buttons+the squared searchbox of El Capitan/Yosemite on window and Spotlight and checkbox Buttons.I hope you enjoy, the guide of how to install is inside the Zip.
Upgrade Mac Os X 10.7 To El Capitan
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.