After the last NordVPN update, this add-on wouldn't log in, then after I tried removing and re-adding it, just kept saying I needed to enable it on private windows in order to use it. about:addons showed it was already enabled for private windows, tried disabling that option and then removing the addon, then added it back and enabled it again for private windows. Seems to be working for now, this issue will probably come back with the next NordVPN update.

*edit* Such a wonky app. This time my PC was asleep, after waking the main NordVPN app didn't reconnect on its own even though it's set to, then the Firefox add-on was stuck on a spinning circle trying to log in. The last time something like this happened, going to the settings (gear icon) on the add-on and logging out was able to fix it, but this time it was completely stuck. Removed the add-on from about:addons in Firefox, restarted Firefox and then reinstalled it again. Had to log back in after enabling it on private windows, but it's working now. We'll see for how long.