Though I hear Mullvad has its own Firefox fork now

Mullvad Browser? It’s basically Tor Browser without the Tor, meant to be used with a VPN instead. It has integration with Mullvad VPN by default, but that add-on can be removed, and the browser doesn’t need Mullvad VPN in order to function. Since the browser is based on Firefox, someone might want to fund Firefox development by using it with Mozilla VPN instead.

After playing with it, it seems that it’s only possible to embed images, not .md files or other text.

Some do! Or at least, they give you a choice of OS at different price points. NovaCustom, Eurocom, and AVA Direct come to mind. Of course, there are also plenty of vendors that ONLY offer GNU/Linux pre-installed…