Unable to upload update - "You cannot submit this type of add-on"

I have recently updated my add-on to work with Thunderbird again since it now has WebExtension support. However, now I am trying to upload the common version to AMO it’s being rejected with “You cannot submit this type of add-on”. The validation report only has an expected warning, so I’m not sure what’s wrong.

Now, reading between the lines, I suspect it’s because I had to add an “experiment” to get a feature working in Thunderbird, and I think maybe the package signer doesn’t like that (although the linter is happy with it in the manifest, and the experiment isn’t actually triggered under Firefox - not that it can know that).

Is that the culprit? I’d really rather not have to maintain two branches for FF & TB that are otherwise identical.

You’re right, AMO won’t allow uploading files with experiments, so unfortunately it won’t be possible to use the exact same file for both applications.