"The current way add-ons are developed gives add-ons complete control over almost anything in the browser. This makes for very powerful add-ons, but add-ons can also do really bad things (accidentally or on purpose) and we can't make major changes to the browser without breaking all sorts of add-ons (which makes people sad).
Web Extensions is a sort of building block set. It means add-ons can't touch anything in the browser, but can only play with the blocks we provide. We can make all sorts of blocks of different shapes, but it will never be as powerful as the old system of add-ons (where developers could play with anything in the house). However, this lets us do major changes to Firefox without having to worry about breaking add-ons (since we know what the blocks are), and keeps add-ons from doing bad things (accidentally or on purpose)"
[1] https://www.reddit.com/r/firefox/comments/6cw7ig/why_is_fire...
So safer for the end user, better for the Mozilla developers but not so powerful for the 3rd party developers.