That every time I have a problem with the following link this always helps

Good add ons

Bin neugierig was dieses Add on macht und kann.

edit: Settings bug was fixed. Thank you.

Thanks! Tiny, self-explaining, yet powerful and comfortable.

this is a very good user agent switcher for Firefox 57* It works great. I would say, it is the best, because i can add cusom user agents. Thank you