The manifest.json-File from version contains a comment in line 17:
"id": "{847b3a00-7ab1-11d4-8f02-006008948af5}", // Enigmail GUID
My python parser does not read JSON with comments, as they are not part of the language, although a lot of tools do accept them.
(https://stackoverflow.com/questions/244777/can-comments-be-used-in-json)
I can fix my problem with the python tool not accepting comments, but I'd suggest anyway to remove the GUID-Comment.
Comments in manifest.json files are explicitly allowed by Mozilla. See the following link:
https://developer.mozilla.org/en-US/docs/Mozilla/Add-ons/WebExtensions/manifest.json