"Changes to default home page and search preferences, as well as settings of other installed add-ons, must be related to the core functionality of the add-on. If this relation can be established, you must adhere to the following requirements when making changes to these settings: The add-on description must clearly state what changes the add-on makes. All changes must be ‘opt-in’, meaning the user must take non-default action to enact the change. Uninstalling the add-on restores the user’s original settings if they were changed."
Comments
And specifically, Mozilla's review process includes a "No Surprises" principle that covers cases like this one:
https://blog.mozilla.com/addons/2009/05/01/no-surprises/
"Changes to default home page and search preferences, as well as settings of other installed add-ons, must be related to the core functionality of the add-on. If this relation can be established, you must adhere to the following requirements when making changes to these settings: The add-on description must clearly state what changes the add-on makes. All changes must be ‘opt-in’, meaning the user must take non-default action to enact the change. Uninstalling the add-on restores the user’s original settings if they were changed."