Firefox solution: “Your add-on failed validation – manifest.json was not found”

If you have developed add-ons for Firefox and ran into the „manifest.json was not found“ error – there’s an easy solution for this! To publish a Firefox add-on them after successful testing, you may need to pack your add-on. Even if the unpacked add-on works locally (and manifest.json was found), it might not work after uploading it as a ZIP archive.

 
Your add-on failed validation with 1 error. manifest.json was not found

The solution to get rid of the add-on validation error is quite simple, as the Firefox developers write in a blog post:

The packed add-on can be in .zip, .xpi or .crx format: For the most common ZIP format, it is important to compress the individual files together in one archive but not add-on folder itself: You need to open your add-on folder, select all the files together and create a ZIP archive (copy them all in one ZIP) to upload to the Mozilla servers.

This is an important difference from plugins and extensions in Chrome, Edge and Opera: These will accept data inside a folder, whereas Firefox will display a „manifest.json was not found“ error.