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Users of an extension get automatic updates unless explicitly turned off on the extension's option's page. Certain extensions, such as those that charge for certain updates, would instead like to have their manifest files initially turn this off, to stop users being up for unwanted or unexpected charges unless they follow a suggestion to manually disable updates.
One solution would be to add an "auto_update_default" field to the "gecko" section of the manifest.
A better solution would be to allow update json files to specify which versions can be safely auto-updated, and which require user confirmation.