Try to go into your Applications folder, deleting Firefox and replacing it with a fresh version from the [https://www.mozilla.org/en-US/firefox/new/ Download page].

''chriss33 [[#question-1230655|said]]'' <blockquote> my macbook air has a pop up saying firefox is critically out of date. when I try to update the download will not replace the existing 59.0.2 </blockquote> Even though your version of Firefox ''is'' out of date : Just to be on the safe side and make quite sure that you're not dealing with a fake update - would you please take a look at this article : https://support.mozilla.org/en-US/kb/i-found-fake-firefox-update