AMO:FAQ
How do I enable the sandbox?
Please see Enabling the Sandbox.
What applications and versions are supported?
Please see the complete list here.
How do I delete my add-on?
Due to security concerns, it is not possible to delete an add-on once its GUID has been submitted to AMO. Instead, you can "disable" your add-on which will hide it from searches, display pages, and automatic updates. You can disable your add-on by clicking the Disable button on the Edit Add-on page. You can go back at any time and re-enable your add-on from the same page.
Why aren't add-on updates sent out for sandbox'ed add-ons?
Due to security concerns. Add-ons in the sandbox are unmoderated. If automatic updates were enabled, then this could be used for malicious purposes. Going "public" forces a review by an editor and lessens the chance of a rogue add-on doing damage thru automatic updating.