I never used Waterfox but Librewolf seems to have more stricts settings, whereas people describe Waterfox as being closer to Firefox.
Librewolf has sensible but annoying default settings that you have to change. For example, cookies are deleted when quitting, or you can't have night mode out of the box since it could be a privacy issue. IMHO it's a cleaner Firefox and I enjoy it so far.
I contributed UX for a 'save cookies for this site' dropdown feature in the navbar, a poc was made which looked good, but it got lost in other work and eventually didn't land in a release that I'm aware of. Shame because that one feature would make it practical to use the recommended clear cookies behavior by default except for particular sites and overall boost everyone's privacy and security because I'm pretty sure most people turn it off after getting sick of logging in. After a couple of months I went back to Firefox and hardened it making it basically the same as LF but not being a month behind in updates. I guess I'll revisit the project now
I created a post[0] on /r/LibreWolf to discuss usability tweaks for new LibreWolf users who might not know all the ins and outs of the more strict settings.
Librewolf has sensible but annoying default settings that you have to change. For example, cookies are deleted when quitting, or you can't have night mode out of the box since it could be a privacy issue. IMHO it's a cleaner Firefox and I enjoy it so far.