this addon has helped me with many tests. thank you for this.

This is a good estension because it spoof others navigator values in about config. I found a bug, after choice a browser prefecence do not update the vendor value. Some browser like Google Chrome have a vendor "Google Inc." or Opera Browser have vendor "Opera Software ASA", firefox have vendor empy "". I suggest to add the spoof of vendor like others navigator values. This is one of few estension that really spoof in serious mode because there are many value to change not only useragents to spoof a browser. Take into consideration the idea to change the value of vendor because it is important and some browsers communicate it to the sites to be identified. Have a nice day.

Update 06 December 2018.
1) I found some incorrect value in this estension, I added the complete image of all navigator values for firefox and Google Chrome, please adapt the estension to show correct value because some site can block browser with incorrect navigator data.
https://imgur.com/60y4pBI
https://imgur.com/PYMcFRS

2) There are a bug, when you choice some useragent must be show oscpu null "" otherwise show original os in the machine. The user agent setted show a user agent (in my foto android) but oscpu (in google chrome is always null "") show real os (in my chase windows 7).
ThIs is a bug important to fix because show a conflict of the os from the user agent setted and oscpu grabbed via javascript.
https://imgur.com/jyfmyCk

I look in the estension there are a javascript file to set the navigator to show after choice an user agent, please look the 2 image added to adapt the estension to show correct values, i added image from firefox and google chrome with complete navigator list values.

Thanks your work.

Bloated and slow. Another addon, "User Agent Switcher and Manager", which do exactly the same, is faster and more than 20 times smaller!

Dude you just stole my heart with this addon. This is working perfectly as I wanted to. I want to add more User-Agents and OS and others for that I need your kind help. I have emailed you to the support email listed for this addon page.

*** I'm seeking for your attention dear admin of this addon sir ***

I need your kind help to add more User-Agents, OS, Versions etc.

Can you publish an Updated Version soon? Or else You can help me to add manually?
I'm really not aware of user-agents for different browsers and OS.

*** Waiting for your response ***

Unlike some other apps I saw suggested from Firefox & others results from Apps search & Google search this 1 actually was to UPLOAD images to certain sites like Instagram (90% of the others were to download images already posted on sites by other people instead of using screenshot apps, almost complete opposite of what I was searching for). And even better you explained how it works & how to use it RIGHT in certain internet servers. That helped a lot more than what I got from Google (lol. Some results were even outdated/impossible to use now). This saved me time & even money because now I don't have to go buy a smartphone just to actually/fully use a website (or 2+).

Seems to be one of the more frequently updated UA changing extensions. Keep it up!

Love it that you can add custom strings (just use Ctrl + V to paste). It works very well. It doesn't fool the Firefox add on site, but I see this is a Firefox policy, and the developer has provided a work around. This is a terrific, easy to understand and use add on. It does fool Google and all the user-agent testing sites.

Tested this addon against sites like: https://panopticlick.eff.org/
As you will clearly see when you test it for yourself, it does not work.
Set it to windows / IE : here is my test result
User Agent
7.92

242.32
Mozilla/5.0 (Windows NT 6.1; WOW64; Trident/7.0; rv:11.0) like Gecko
as you can clearly see it does not work.

Missing the "append" mode and a confirmation check when you delete a existing user-agent string.
Apart from that, its fine and simple.