I think we chatted about this over matrix, but the issue is about host permissions, in MV3, they’re not granted automatically on install, but instead user needs to grant them at some point.
Depending on what’s appropriate for your extension, you could ask the user using promise.request() , or they can grant it by going to about:addons, selecting your addon, and flipping the toggle in the permissions tab.
We’re also working on an easier UI to do this, tied to your extension browser action, but that’s not final yet. Thanks for pointing out that this is not clear from the migration guide, I’ll work to get that updated with this info as well.