We have been reported that users experiencing "Secure Connection Failed" message when performing various actions on web (incl. eSales) using Firefox browser.
This is decleared as a bug by Firefox and introduced on version 46x. The bug info can be tracked on this page: https://bugzilla.mozilla.org/show_bug.cgi?id=1269055
Our suggestion to users who have this issue is to either use a different browser, or downgrade your Firefox.
You can also check our posted announcement for the certified/compatible browsers on the portal.
It appears the reported bug will be fixed on upcomming Firefox version 47. Firefox will release version 47 when they feel it is stable enough for release, and at that time you could upgrade your Firefox without issue.