I'm also having the experience of it working well up till recently. (A week or more ago?) Now it seems to be giving wrong codes or something? I had to go back to phone based Google Authenticator. (those numbers aren't the same as the ones in the plugin for some reason, they should be right? at the same time?) Firefox 65.0.1 Mac
Used to work fine, but since update I can not use and am locked out of the sites I used it for..
19/02/19 - I don't seem to be able to reply to your comment below so have edited this post instead.
I have no idea which update, I had not used my laptop for a while. All I know is when I tried to use this it says there are no sites configured. I am unable to access the sites I used it for. When I attempted to roll back to a previous version it removed the add-on completely from my browser. I am using Firefox 65.0 atm.
Really great extension, thanks for it :-) Just a little thing I think should be improved is about color of fonts when using a dark theme, because black on nearly black, it's not easy to read ;-) For the rest, works like a charm. Keep the good work :-)