UserAgent: Mozilla/5.0 (X11; Linux x86_64; rv:54.0) Gecko/20100101 Firefox/54.0

Steps to reproduce the problem:
1. Try and fail many methods of communicating synchronously to the page in a web extension...

What is the expected behavior?

What went wrong?
Web Extensions have no way to reliably inject user-configured content (such as userscripts) on to the page a document_start. This severely hampers extensions that require their scripts run absolutely first before anything else on the page.

For reference, Firefox is creating a new method called "registerContentScript":
https://bugzilla.mozilla.org/show_bug.cgi?id=1332273

Did this work before? No 

Does this work in other browsers? Yes

Chrome version: 59.0.3071  Channel: stable
OS Version: 
Flash Version: Shockwave Flash 11.2 r202