Thank you for your reply, Anshu.
My computer has the latest version of Windows 10 Professional
I am using what you call the traditional or web version of Enpass, version 5.6.0.
My Firefox 57 enpass browser extension version is from your browser extension download page, version 126.96.36.199
I use BitDefender Total Security 2018 on my computer, and BitDefender's firewall setting for Enpass allows all access -- Any Protocol, Any Traffic, Any Port, Any IP
Question: am I using the correct Enpass version and the correct browser extension for FireFox 57?
If so, then I need to tell you Enpass is NOT working for me, because I am still getting the Enpass Connection Error message.