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Where can I find a full Firefox/Mac OS compatibility list?
What versions of Firefox will work with what versions of Mac OS?
What versions of Firefox will work with what versions of Mac OS?
Wšykne wótegrona (1)
https://support.mozilla.org/en-US/kb/firefox-osx
Firefox 49.0 and later requires OSX 10.9 or later now.
Those still using the old OSX. 10.6, 10.7, 10.8 can use the Firefox 45.9.0 ESR to be up to date security wise for a little while longer. The Fx 45.9.0esr came out when Fx 53.0 was Released and is likely the last for the old 45 ESR.