Вот цитата из MDN (публиковать ссылки в отзывах запрещено, так что используйте поисковики):

> Note: Changing proxy settings requires private browsing window access because proxy settings affect private and non-private windows. Whether an extension can access private browsing windows is under user control. See Extensions in Private Browsing for details. Your extension can check whether it has access to private browsing windows using extension.isAllowedIncognitoAccess. If your extension doesn't have private window permission, calls to proxy.settings.set() throw an exception.