Almost unusable on Firefox while on other browsers it works okay. There are 3 MAJOR issues on Firefox:
1. When you click on extension icon it takes around 3 seconds for UI to show up. This is extremely annoying even more because of other bugs that are forcing you to interact with extension more then you need to (will explain it now).
2. When you finally connect to server and after you restart Firefox NordVPN icon will be green which indicates that proxy connection is established. However that is not true, you are not REALLY connected although extension is suggesting that. You have to click on icon (and wait 3 seconds), disconnect (3-5 seconds) and finally reconnect (3-5 seconds). So if you want to use this extension on Firefox, you have to do this every time you start Firefox.
3. Extension won't even open UI in private mode.

This is a paid service and this extension is almost unusable, forcing me to leave 1 star review. I wouldn't recommend it even if it was free, at least not on Firefox.