ProtonMail Extension is described as 'Unofficial ProtonMail extension. Provides notifications and makes it possible to close the ProtonMail tab, and even the browser, without getting logged out' and is an app in the Web Browsers category. There are three alternatives to ProtonMail Extension for a variety of platforms, including Linux, Mac, Windows, Online / Web-based and ProtonMail. The best alternative is ProtonMail, which is both free and Open Source. Other great apps like ProtonMail Extension are ElectronMail (Free, Open Source) and Bridge (Paid).
ElectronMail is an Electron-based unofficial desktop client for ProtonMail and Tutanota end-to-end encrypted email providers. The app aims to provide enhanced desktop user experience enabling features that are not supported by the official in-browser web clients.