Hi, as Firefox is not a Email Program how does this have to do with Firefox. You have not given enough information. If McAfee cleaned something out of Firefox have it restore it. At this time McAfee is not being Supported by Firefox for these same issues.

There is security software like Avast/AVG, Kaspersky, BitDefender, ESET and sometime McAfee that intercept secure connections and send their own certificate and or block things with their scanners. If you are running any of the above software please check their community forum for a work around to apply settings. *https://support.mozilla.org/en-US/kb/secure-connection-failed-error-message#w_avast-and-avg-security-products

''Bigheap [[#question-1234870|said]]'' <blockquote> I use Windows 10 and McAfee security. I have tried disabling McAfee but it doesn't help. I can send emails but not receive them </blockquote> Maybe you could follow [https://service.mcafee.com/webcenter/portal/oracle/webcenter/page/scopedMD/s55728c97_466d_4ddb_952d_05484ea932c6/Page29.jspx?wc.contextURL=%2Fspaces%2Fcp&articleId=TS100834&_afrLoop=117201604517106&leftWidth=0%25&showFooter=false&showHeader=false&rightWidth=0%25&centerWidth=100%25#!%40%40%3FshowFooter%3Dfalse%26_afrLoop%3D117201604517106%26articleId%3DTS100834%26leftWidth%3D0%2525%26showHeader%3Dfalse%26wc.contextURL%3D%252Fspaces%252Fcp%26rightWidth%3D0%2525%26centerWidth%3D100%2525%26_adf.ctrl-state%3Ddrkjdl29m_9 these steps]