With recent changes, addons.mozilla.org (AMO) now more strictly enforces our rule requiring version numbers for add-ons to be unique.
Previously, the website allowed some edge cases where if a version was deleted or disabled before we reviewed it, a new version with the same version number could be uploaded. This caused issues with our CDN where the old xpi was cached. The hash (used for verification) could be wrong so the install would fail, and any users who installed the old xpi would fail to get the update to the ‘real’ version.
Now, once a version has been uploaded to AMO with a version number, you will be unable to use it again. Deleting or disabling the version won’t release the version number for re-use. This restriction applies to add-ons that are uploaded for signing and external listing as well as ones listed on AMO.
We’re aware this will cause some irritation to developers when they try to upload again, and that perfect version “10.0.0.0” may be lost due to a hasty upload, but we believe the user benefits in this case outweigh the extra restriction on developers.