Doesn't work on Linux.
EDIT: Got it to work by removing the extension and adding it again. I think the extension needs to be installed AFTER installing the app. Installing the app afterwards did not seem to fix the non-working extension.

I rated this 5 stars. Because it works on Android Firefox and Desktop Firefox. This is a feature I need and therefore cannot use Lastpass or other competitors. As far as I can see 1Password has best cross-platform/cross-browser/cross-device. This is such a good feature. Thank you. You have earned a paying customer after my trial is done.