This is a good question.
The Free Software Foundation does not include Ubuntu in its list of free GNU/Linux operating systems.
Their reasons are:
Ubuntu provides specific repositories of nonfree software, and
Canonical expressly promotes and recommends nonfree software under the
Ubuntu name in some of their distribution channels. Ubuntu offers the
option to install only free packages, which means it also offers the
option to install nonfree packages too. In addition, the version of
Linux, the kernel, included in Ubuntu contains firmware blobs.
Ubuntu's trademark policy prohibits commercial redistribution of exact
copies of Ubuntu, denying an important freedom.
Don't worry, though! There is a free version of Ubuntu which has actually been endorsed by the FSF: Trisquel.
Trisquel meets all of the FSF guidelines for free system distributions. It is based on Ubuntu but is without the non-free repositories and it uses the Linux-libre kernel. Not so many people know about Trisquel, unfortunately. Tell your friends about it! Try it out yourself.
Trisquel is under active development and has an lively and helpful community.