The Firefox 45.X.0 ESR updates are major updates for security and allowed stability fixes in line with the Major Firefox Releases. The current Release is actually 45.'''6'''.0 esr as it came out when Firefox 50.1.0 was released. You can do a update in Firefox or download from https://www.mozilla.org/firefox/organizations/all/ https://www.mozilla.org/security/known-vulnerabilities/firefox-esr/ https://www.mozilla.org/security/known-vulnerabilities/firefox/ As you are probably aware Firefox 49.0 and later requires OSX 10.'''9''' or later now. https://support.mozilla.org/en-US/kb/firefox-osx Think of the version as say forty-five point six point zero and not forty-five point sixty