I have personally used fedora and nixos on a gen 1 framework 13 and it works great.
Does Framework do anything regarding FOSS drivers or firmware?
Regarding your question they say this:
We deliberately selected components and modules that didn’t require new kernel driver development and have been providing distro maintainers with pre-release hardware to test to improve compatibility. We’re also working on enabling firmware updates through LVFS to complete the Linux experience.