Is there a way to use Threema on a Ubuntu desktop?

1. Threema for Desktop (since Oct 2021, an Electron-App), as a deb-package. Outlook: Threema 2.0 for Desktop is worked on, with the multi-device technology.

• Helpful tool: Session password
  Enter a session password in the desktop app or the web client before scanning the displayed QR code with your mobile device. To use the desktop app or the web client the next time on the same computer, you can simply enter this password (instead of scanning the QR code again).
  This way, it’s also possible to use Threema if your mobile device isn’t present (provided it’s connected to the Internet).
  But hopefully Threema 2.0 for Desktop will come soon!

3. Threema Web (web.threema.ch) is Threema’s web client. It allows you to use Threema on the desktop without compromising security. By scanning a QR code, desktop and mobile device are connected at once, and all conversations are fully accessible on the desktop. More details on the Threema webpage.

4. openMittsu is a cross-platform open source implementation and desktop standalone client for the Threema Messenger App. More infos on the openMittsu Github page.
   You can use the repository, or install the packages directly.
   If you like to use the repository, add the GPG key for the repository:

wget -O- https://packages.openmittsu.de/openmittsu.packages.gpg.key|apt-key add -

Now, add the repository. As root or using sudo, create a file called openmittsu.list in /etc/apt/sources.list.d/ with the following contents:

deb https://packages.openmittsu.de/apt/debian stretch main

You can now install (or update) openMittsu using:

sudo apt-get update  
sudo apt-get install openmittsu

4. Ferdi, messaging browser that allows you to combine your favourite messaging services into one application - free & open-Source alternative to Franz & Rambox.
   You can install it as deb, AppImage or Snap package.

5. Rambox, an Electron based Free, Open Source and Cross Platform messaging and emailing app that combines common web applications into one.

   Threema is among the available services along with Wire etc. There is also an option to add services manually.

   To install Rambox, first download the deb package right for your system from their official GitHub page. Then run the following command in Terminal

sudo dpkg -i /path/to/the/package/package-name.deb