Works great for me on desktop firefox. Note that some websites might use other tricks to stop you from selecting text, and its not the fault of this extension. The extension only blocks the CSS method.

No effect in Android. :(
Type also: unblock Copy protection in the addon Info for better finding

I love it. Now I can copy paste from Japanese grammar sites without getting a hernia.

Android works, but does not work on Windows

На Android работает, а на Windows не работает

Works as described on Android after restarting the browser.

You must restart your browser for the changes to take effect.
I gave it four stars because I just installed it,
and I haven't yet been able to test it thoroughly.

It doesn't work on Android!

Works. But if you wanna know how it works use Stylus extension and go to https(:)//uso(dot)kkx(dot)one/browse/styles and search for user-select blocker

Funktioniert nicht.

FF on Android. Allows text selection on comments on YouTube dot com without any hassle. thanks.