FF extension doesn't work: An unexpected property was found in the WebExtension manifest.

[immwi]

I followed the compilation steps and when loading the zip-file FF says, the extension seems to be damaged.
Reading manifest: Warning processing version_name: An unexpected property was found in the WebExtension manifest.

[immwi]

after removing the line
"version_name": "1.1.1-firefox",
in shells/firefox/manifest.json it still doesn't work. The error is gone though.