extension works well. speed is acceptable. but it's not compatible with some other popular extensions "Firefox Multi-Account Containers" for example and some others. I had to disable 4 extensions before it started working for me. And yes I used all 4. So now it's quite a headache to switch between extensions.
btw their openvpn version on Linux doesn't work at all. A chance I can load anything with it is close to 0.
