Note Firefox 49.0 and later will require Mac OSX 10.9 or newer. So for OSX 10.6, 10.7, 10.8 users you can then switch to the Firefox 45.0 ESR branch to still get security updates for Firefox. It will be supported until 45.8.0esr which is when Firefox 53.0 will be released at same time. It is possible Mozilla may decided to extend life of Firefox 45.0 ESR past 45.8.0esr both due to Mac OSX 10.6, 10.7, 10.8 support being dropped and also due to not supporting older cpu's that does not support SSE2 starting Firefox 49.0 also.Dick Guertin wrote:To all following this thread: I've updated Firefox for Mac on OSX 10.6.8 and 10.10.5 to version 48.0, which is last update to 10.6.8 available. Any future update will have to be of the 45.3.ESR version.
https://support.mozilla.org/en-US/kb/firefox-osx