Running an out-of-date browser is a security concern, is it not? Does Debian have a policy where they'll just backport the latest non-ESR from Unstable whenever a new 0-day is discovered? If they do and they're quick about it, I guess it's more a matter of personal preference.
Anyway, using Guix, Nix, or Flatpak is definitely a better idea than installing standalone debs for a different version of Debian than you're running, where it may be built against libraries that aren't part of your distro.
If you want to use the Debian package from Sid on Testing, better to rebuild it from source. openSUSE's public instance of the Open Build Service is a convenient way to manage keeping a repo for that up to date. But that definitely complicates system maintenance, too!
Anyway, using Guix, Nix, or Flatpak is definitely a better idea than installing standalone debs for a different version of Debian than you're running, where it may be built against libraries that aren't part of your distro.
If you want to use the Debian package from Sid on Testing, better to rebuild it from source. openSUSE's public instance of the Open Build Service is a convenient way to manage keeping a repo for that up to date. But that definitely complicates system maintenance, too!