If the manifest validator was a standalone application, where I didn't need to upload a full version of a packaged app that's a few hundred MB, I could have a much faster turnaround time figuring out why manifests are failing to validate. If there was a simple RESTful API, others could consume this service as well, such as webIDE.
There is a RESTful API, but you do need to POST the package to it, so it's going to take a long time. http://firefox-marketplace-api.readthedocs.org/en/latest/topics/submission.html#validate Alternatively, we should improve the docs but you can download the validator as a standalone app, there are instructions over at https://github.com/mozilla/app-validator/
There is a JS validator in the works and the plan is adding that into Web IDE. Details on that are here: https://wiki.mozilla.org/Marketplace/Validator
Based on the API and the JS validator, I think we have this covered.