I have played Xonotic[0] for some time and it's a lot of fun. As far as i know it's entirely libre.
Honestly I think it's probably the best libre FPS out there. I wish it was more popular, but considering how poorly the new Unreal Tournament did, I honestly think the general gaming community isn't interested in quake style games anymore.
Another sort-of-game I like is AlephOne, which is an open source implementation of the engine for Bungie's Marathon games. I believe Bungie released all the marathon assets as "free for non-commercial use". There's also some very fun open source campaigns on it.
I've been playing Xonotic for many years, but there's some things that I believe are holding it back. Same goes for other libre FPS games we have like Alien Arena, Sauerbraten, Tesseract, Red Eclipse, Warsow/Warfork ... If you're interested, here's a video where I discuss that: https://youtu.be/JAQH2AVvKhU