Nothing lasts forever, including browser support and operating systems. Apple has already kicked OS X versions 10.8, 10.7 and 10.6 to the curb and in August this year, Firefox on these platforms will also be given its marching orders by Mozilla.
The announcement was made via Mozilla's Future Releases blog, where it's explained that the company will no longer support Firefox on the aforementioned operating systems from August 2016 onwards:
At that time, Firefox will continue to function on these platforms but will no longer receive new feature or security updates ... All three of these versions are no longer supported by Apple. Mozilla strongly encourages our users to upgrade to a version of OS X currently supported by Apple.
There is an exception — Mozilla plans to uphold support for Firefox 45's Extended Support Release on OS X 10.8 / 10.7 / 10.6, though only until "mid-2017". Yep, that sounds like plenty of time to upgrade your browser and OS if you haven't already.