I just tested your extension with this site: https://www.deviceinfo.me/ and surprise it's not that good.
Here is the response from the site "Supports real browser kernel detection even if the browser is spoofed, for: Firefox, Chrome, Edge, Internet Explorer, Safari, Opera, Konqueror."

Well that looks like to be it with YouTube as well.
I can't get it to deliver the classic YouTube any more on anything there when set to Google Bot.

YouTube sure wants to force that completely ugly polymer "tablet look" of theirs down peoples throats, even though they might risk loosing a ton of computer users because of this.

Used to work well but Is now breaking sites. Please fix.

no me gusto al no tener una lista de agentes antiguos pues esa es una funcion muy buena por ejemplo IE6

ok

it doesn't work after i updated Firefox

I needed it for a particular website that only wanted to support Chrome. I downloaded this, installed it, and picked the only Chrome user-agent option (Chrome 59). The site then assumed I was on Chrome, but complained it was out-of-date so I couldn't use it. I found no way to use a newer user-agent, so this add-in does me no good.

Very inconvenient way of importing the user agent in text form.

Hello! I decided to use your supplement. But the first thing I did not like was the lack of the ability to auto-update user agents. I thought I could solve this problem by manually adding agents through the site https://techblog.willshouse.com/2012/01/03/most-common-user-agents/ On this site there is a string display of agents for example:
"Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/60.0.3112.113 Safari/537.36
Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/61.0.3163.100 Safari/537.36"
But when I began to add them, they were wrong. It turned out that these lines should be edited and indicate which client is available in fact - dekstop or mobile ... It's not convenient! Is the add-on not able to determine whether the agent belongs to the OS on the line "... (Windows NT 10.0, Win64, x64) ..."?

Doesn't save the agents wich have been added by the user.