Slightly OT: There is another problem that most people seems to miss. Add-ons in their current form in ff are tightly coupled to the browser to the point that it's the browser itself. In the modern web, the browser knows too much about you, and so does the add-on, and it's not unheard of them to explore it[1]

How does that free webpage to PDF converter add-on make money? By selling your web history of course.

[1] https://www.google.de/amp/thehackernews.com/2016/11/web-of-t...

EDIT: added OT tag