I'm also having this issue. I don't seem to have another Enpass and rebooting doesn't help. I'm running Mint 19.1 Cinnamon. When I install enpass, I get a menu item for Enpass. Choosing the menu item doesn't run enpass. I can start enpass from /opt/enpass. Setting up sync takes me to the Google login page and - after I approve - prompts me to open xdg-open. The process never completes, however. Would appreciate any ideas!
I have the same issue - running Ubuntu 16.10 / FF 50.1.0 / Enpass 5.4.1. The browser integration stopped working in the last few days, I think with the Enpass upgrade.