Right now it's checking both hidden and visible checkboxes: https://searchfox.org/mozilla-central/rev/82828dba9e290914eddd294a0871533875b3a0b5/browser/components/migration/content/migration-wizard.mjs#695-697

It should only check the visible ones. That way, we don't accidentally confuse the parent process with a request to migrate things for which there are no resources.

At the same time, we should also make the MigrationWizardParent more resilient to this, where it will choose to ignore any attempts to migrate resources that it doesn't support.

STR:

1. Make sure browser.migrate.content-modal.enabled is true and browser.migrate.content-modal.import-all.enabled is true
2. Open the Migration Wizard (about:preferences, choose "Import Data" from the General section)
3. Choose any browser from the list (I chose Safari on macOS)
4. Expand the details section for the various resource types, and click "Select All" to de-select everything
5. Click "Select All" again to reselect all of the checkboxes
6. Click "Import"
7. Depending on whether or not you've already given permission to the migrator, you might have to get through the permission flow, in which case, do that.

ER:

Once the migration begins, we should see just the checked items were imported from. The "Done" button is enabled.

AR:

"Form autofill data" gets listed even though it wasn't selected as a resource type, and shows a progress spinner. The "Cancel" button is disabled. See screenshot.

This also adds some belt-and-suspenders checks here so that even if the Migration Wizard
somehow gets confused and sends resource types that the migrator can't import from, they'll
just get ignored rather than having a forever-progress-spinner for that resource type.

https://hg.mozilla.org/mozilla-central/rev/29aea095b9be