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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. Firefox 49.0.2.dmg Mozilla Firefox for Mac is a fast, full-featured Web browser. The app includes pop-up blocking, tab-browsing, integrated Google, Yahoo and Bing search, simplified privacy controls, a streamlined browser window that shows you more of the page than any other browser and a number of additional features that work with you to.

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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If you're using an outdated version of Firefox on Mac OS X 10.8 (Mountain Lion) or earlier, the only way you can update Firefox to a secure version is to upgrade your Mac OS to a version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.
Firefox 49 Download For Mac
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.