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.
Thanks for the review! Unfortunately this is a limitation of the WebExtension API: The Firefox add-on site is blacklisted from manipulations of extensions due to browser policy. I've added this (and a manual workaround) to add-on's description.