Libreboot use to be part of the GNU project, but I'm not aware of any collaboration after the fact. I just know the primary author/maintainer of Libreboot had a falling out with the FSF and GNU project. I don't think they ever reconciled (and I'm not sure they will at this point).
As for the cause of their falling out, this happened around 2016 when the FSF let go of a transgender employee. It appears the original statements by the Libreboot maintainer was deleted off of their site, but they are archived[0][1] and the original email sent out declaring themselves no longer part of the GNU project[2]
Just to be clear, I am not sure if an actual reason was given out for why they were fired. The FSF had declined giving an actual reason, and Richard Stallman himself stated "The dismissal of the staff person was not because of her gender. Her gender now is the same as it was when we hired her. It was not an issue then, and it is not an issue now."[3]
Those articles and emails are the only primary sources that I'm aware of. There might be more information somewhere else, but unfortunately I don't really know of anything further.
There was more drama later on. Something about the author leaving the project and then forcibly taking it back a few years later down the line against other contributor's wishes. But I think the reason for that was a bit more nuanced and I didn't keep up with that. I'm not even sure it had anything to do with the FSF or GNU project anyways.
[2] https://lists.gnu.org/archive/html/libreboot/2016-09/msg0003...
[3] https://lists.gnu.org/archive/html/libreboot/2016-09/msg0005...
