I have noticed that storage.local.get API works differently in Safari 18 compare to other browsers and Safari 17. browser.storage.local.set({abc: 123}) browser.storage.local.get(console.log) // {abc: 123} browser.storage.local.get({abc: null}, console.log) // expect {abc: 123} but got undefined instead. According to MDN, storage.local.get should work with an object with object names as keys and the default value as value. According to Safari 18 Release notes, this may be related to a fix: 'Fixed getting an empty key from storage. (99440265) (FB11427769)'.