Why SCP is blocking script-src resource on firefox version 77.0 or later?

I have an add-on that changes csp headers and it just stopped working on firefox version 77.0
Before that version I didn’t have any problems.
Don’t know why really.

To demonstrate the issue, below you will find a simple add-on I developed.
If you run it on a page with csp on firefox version 76.0 it works fine.
If you run it on firefox 77.0 or later you get this error:
Content Security Policy: The page’s settings blocked the loading of a resource at inline (“script-src”)

Here is the link to the add-on:

Any help would be appreciated!