
Please, read carefully:

manifest.json is the right Web Extension manifest file for Chromium 
based browsers like Google Chrome, Microsoft Edge, etc.

Due to the leak of Mozilla Firefox to support "sandboxes" Web Extensions,
manifest_firefox.json is specific for the Mozilla Firefox browser.

When you wanted to debug or deploy the Web Extension in Firefox, you need to 
use the manifest_firefox.json, by renaming it to manifest.json
