Just recently I have started having a problem with Enpass on my iMac with Firefox set as the default browser.
My iMac has the latest version of Mojave, the latest version of Firefox and the latest version of Enpass. Enpass is also installed on a PC runnng Windows 10 with Firefox as the default browser and also on two iPhones and an iPad. All are synchronised with the latest version of OneDrive.
I do not Enable Browser Extensions on Enpass it is used as a standalone manager.
If I make any changes to the iMac installation of Enpass it fails to sync. If I then select 'Resolve Now' and 'Merge' the synchronisation takes place and is reflected correctly on all of the other devices. If I make Safari the default browser I don't have a problem.
Are you aware of a specific problem with Firefox on Macs?