NordVPN works very well and seamlessly with Firefox. I do not use it all the time, but when I do it works well. Moreover, it is compatible with my Phone (Android) where I use it most, especially where there is free WiFi (stores, airports, etc.). I also use it on field work in countries where there are restrictions on internet use, for example when browsing for pages that are restricted or not allowed by the government.
I did not want a free VPN since I do not fully trust them and I hate ads. After some research Nord VPN seemed the best one, and I have been happy ever since.

The #1 VPN service available hands down. You can literally be in any country in the world anonymously without hackers or the government sticking your nose in your business. Your internet activity and security is completely safe on all devices.

We using NordVPN from the Island Madeira/Portugal many Times. In Spain or Netherland we used it too. Never had problems with watching German TV on Tablet/LapTop, working with a LapTop, Cellphone on the Internet incl. OnlineBanking, etc..

Simple d'utilisation, fonctionne très bien. Rien à redire

Nice add-on, does what should do.