I haven't been able to try this for any length of time because it's too distracting. It adds a bright green “JS” button to the address bar on *every* website. When you right-click this button, the option “Pin to Overflow Menu” is greyed out.

Se o ícone do addon "JS" não aparecer ao lado do botão "Favoritos", reinicie o Firefox que ele aparecerá.
If addon "JS" icon doesn't visible nearby "Favorite" icon, restart the Firefox. The JS icon will be visible.

Works fine for me so far.

In my experience there are sites that use JavaScript to provide functionality, and sites that use JavaScript for annoying "growth hacking" tricks, such as newsletter subscription forms, on pages that are otherwise regular HTML.

This add-on lets me leave JS on by default but revoke the JS privileges of the "growth hackers."