[aruggirello]

Debian Testing isn't really unstable - the dev wasn't exaggerating. But I'd also suggest Kubuntu (you can remove snap and all of its packages, and install Firefox and Thunderbird .deb's from the Mozilla repo)

[hparadiz]

Debian testing is rolling release. More akin to Arch or Gentoo really.

[tmtvl]

You're thinking of Unstable (Sid). It's also not like Arch or Tumbleweed because it gets locked down during release freeze and then gets a ton of updates all at once.