Update: New version is broken, install 0.2.1 instead.

Allows domain or URL-specific spoofing, exactly what I need.

https://discourse.mozilla.org/t/-/67381/14?u=grahamperrin

The addon works like a charm and it is the most powerful when it comes to change UserAgent.
Definitely a must-have for advanced users in the need of this feature.

However, i believe, it could have a simpler settings' page if one could have a combo box for Platform Selection, another for OS selection and another for Browser Selection, all separated from each other (leading to the final User Agent String).

That would work kind of a "step-by-step wizard" that would avoid the actual very (VERY) long options' combo box that might scare new users...

It only needs to be recursive to be 5 stars, but it must be complex to implement this functionality.