Update:
If firefox updated to 66.0.3 this extension is not working. Update to 66.0.4 and it will be fine.
1Password and 1Password X are great.

Original:
Just like someone else said: the old 1password extension was marked as legacy and when trying to add this one I keep getting an error "check your connection". After I kept trying different things, I found that if I open the link in another tab I get the message: "Firefox prevented this site from asking you to install software on your computer." When I try to reload the link I get "The add-on downloaded from this site could not be installed because it appears to be corrupt." How can this happen??! Why would firefox prevent addons.mozilla.org to ask about installing software? If the reason is a corrupt file that should be the message from the beginning. Most importantly, was this not tested?!?!

If you're looking for the extension for the 'Classic' version of 1Password, follow these instructions from AgileBits: https://support.1password.com/firefox-connection-failure/

Hervorragend. *****