N.E.P.T.R

You can use both through the browser, which is the safest way of doing things because the browser sandboxes the web apps, isolating them from your system. If you prefer an app for Messenger, look on Flathub, though I advise against it. The two apps I found for Messenger are Franz and Ferdium (a fork of Franz with more features).

To mitigate the privacy risks:

• Firefox with uBlock Origin (or Librewolf)
• Avoid sharing anything sensitive.

Nothing much you can do sadly.

It is source available, not open source.