Horrible.
I added it because Avast harassed me about it. It restarted Firefox without any warning and I saw it deletes cookies, which I need. So I uninstalled it.
Since, it's impossible to go on some websites (like FB or Twitch, for example). It charges infinitively.
Even after a restart of Firefox and my PC, it doesn't works. To add at my incomprehension, this morning, Reddit (and some other websites) was totally ok, and this evening, impossible to go there.
This add-on fucked up Firefox so much that even after uninstalling it, it still causes problems.