I unchecked “Query OCSP” and it works fine.
Great add-on. Thank you.

It's very useful but it often occurs that, after few minutes of normal working, suddlendly happens that it's impossibile continue browsing (all web sites unreachable, no connection, not possible restart any vpn tunneling). There's no way to restore the functionality. You can only restart the browser completely and the extension will restarts to work. This is the only way. Quite frustrating.

A little slow navigation

While it's possible to use it in tandem with the Desktop app via split tunneling, the overall experience is frustrating.
Unlike the full version, the Firefox version suffers from intentionally slow speeds (with "a cap of 100 Mbps set on squid services to prevent the saturation of server ports through proxy connections").
NordVPN itself lives up to the hype, but it's better to just use the Desktop version at the moment.