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.
OS X El Capitan on Unsupported Macs macOS Extractor, OS X Patcher, and MacPostFactor are apps that guide you through patching and installing OS X El Capitan (10.11), Yosemite (10.10), Mavericks (10.9), or Mountain Lion (10.8) on your older Mac. This thread focuses on OS X El Capitan. Select the hard disk type. After that, you clicked on the folder icon now in here select the image file of Mac OS X El Capitan and click the “Open” button. Select Mac OS X El Capitan ISO file. In this step, you are done with all the settings it is time to create a virtual machine for Mac OS X El Capitan.
If you're using an outdated version of Firefox on Mac OS X 10.8 (Mountain Lion) or earlier, the only way you can update Firefox to a secure version is to upgrade your Mac OS to a version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.
Os X El Capitan Install
Downgrade Os X El Capitan To Lion
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.