Steps to reproduce:

1. Open Firefox with enpass extension enabled
2. Open site with iframe (like this one)
3. Open browser developer tools (F12)
4. Click in form field
5. Try Selecting the text 'Simple Form' or the lorem-ipsum above the form
6. Check the Console tab of your developer tools for the 'Error: Promised response from onMessage listener went out of scope - background.js:8:59712' message

Workaround:

1. Open Firefox
2. Disable the enpass extension
3. Repeat steps 2-5 above