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 For Mac 10.15
If you're using an outdated version of Firefox on Mac OS X 10.8 (Mountain Lion) or earlier, the only way you can update Firefox to a secure version is to upgrade your Mac OS to a version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 Mac OS X 10.9, 10.10 and 10.11 users move to Extended Support Release; Installing Firefox on Mac. Visit the Firefox download page in any browser (for example, Safari). It will automatically detect the platform and language on your computer and recommend the best version of Firefox for you. To get Firefox for Mac 10.4.11 you need to download Firefox 3.6.28 for Mac which is the last version of Firefox to support PowerPC (PPC) Macs. Mozilla has dropped support for Mac OS X 10.4 but Firefox 3.6.28 still works on PPC Macs.Important Note: Running Firefox 3.6.28 on a Mac poses a serious security threat to your Mac. Mozilla no longer issues security updates for it and it has many.
Firefox For Mac 10.14
Firefox For Os X
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.