Small bug that I noticed.
Using Windows Traditional 5.6.0
Firefox 57 latest extension
Open Firefox
Open Enpass extension: pop-up appears, you can search your passwords or autofill
Don't do anything with Enpass and close the pop-up by clicking outside the enpass window
Open Enpass extension again: the pop-up appears and will immediately disappear
After this encounter, you won't get this bug again untill you close Firefox and repeat above steps again.
It also seems to happen if you have used enpass for some sites, but then decide to do the trick mentioned above, for example:
Open Firefox
Open Enpass extension: pop-up appears
Use Enpass extension to log you in to a site (you can do this for multiple sites)
Use Enpass extension to open the pop-up, don't do anything, immediately close it
Use Enpass extension to open the pop-up again: pop-up appears and disappears again
Make sure if you test this out, that all Firefox instances are closed when you try to reproduce this bug. I'm not able to reproduce it twice during the same Firefox session.
I have no idea if other browser extensions have this bug as well...