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.