Ubuntu GNU/Linux is not entirely FOSS, as it ships with non-free software by default. If you’re committed to FOSS principles, I would recommend Debian GNU/Linux instead.
However, it’s important to note that Debian GNU/Linux is not recognized by the Free Software Foundation (FSF) as a completely FOSS distribution. This is because Debian includes non-free firmware packages for those who need them.
From a security perspective, this is acceptable, as the Linux kernel won’t load these packages unless the corresponding hardware is available. Debian ships with Free Software by default, and I would suggest giving it a try if possible!
It’s essentially the same as Ubuntu, but more freedom-respecting.