Threema's web client allows you to use Threema on the desktop without
compromising security.

* Complete synchronization: By scanning a QR code, desktop and mobile
  device are connected at once, and all conversations are fully accessible
  on the desktop.

* Security and data restraint: All communication between phone and PC is
  fully end-to-end encrypted. After the session is terminated, all
  synchronized messages are immediately deleted in the browser.

* Open Source: Threema Web is open source and uses open standards. It is
  also possible to host Threema Web on your own server.

WWW: https://github.com/threema-ch/threema-web
WWW: https://threema.ch/en/faq/web_info