Why would you want to remove the user agent string entirely, instead of setting a very generic string?

I have been omitting the UA header for decades. Most HTTP requests I send are from the command line so I control what headers are sent. If the header is added automatically by some software I am using, then I remove it with a forward proxy.

IME, the UA header and several others that are included by default in so-called "modern" web browsers are not needed for the majority of servers to which I connect. I only send what is needed to get the HTTP response I want. This approach has worked for me for many years, but it might not work for others.

Of course, today this header is mostly used for advertising purposes. Whether the benefits, if any, for www users (cf. web developers, advertisers, etc.) of sending it outweigh the costs is debatable. Even some so-called "tech" companies see the UA header as of dubious value.

https://www.infoq.com/news/2020/03/chrome-phasing-user-agent...

https://www.zdnet.com/article/google-to-phase-out-user-agent...

As for why this extension author is removing the UA header, it may be something to do with Tor, according to his profile.

https://addons.mozilla.org/en-US/firefox/user/19013409/