Open that app from your Applications folder to start 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 10.10 can upgrade Mavericks, Mountain Lion, Lion or Snow Leopard. Mavericks, for instance, fell to 14% of all editions of OS X, a two-point slide that was double its earlier average. Approximately 90% of all Macs were eligible to upgrade to El Capitan when the operating system launched on Sept. El Capitan will run on the same Macs that have run Yosemite, Mavericks, 2012's Mountain Lion and 2011's Lion.
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.
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If you're using an outdated version of Firefox on Mac OS X 10.8 (Mountain Lion) or earlier, the only way you can update Firefox to a secure version is to upgrade your Mac OS to a version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.
How To Upgrade From Os X Lion 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.