Project website: https://librewolf-community.gitlab.io/
AUR package: https://aur.archlinux.org/packages/librewolf/

I've built librewolf 75.0-1 locally after adding only the following 3 lines to the PKGBUILD from the AUR:
https://git.parabola.nu/abslibre.git/tree/libre/iceweasel/PKGBUILD#n300

Please refer to the project website, docs, and code first.
Any constructive corrections, comments, concerns, criticism, etc. are welcome, because as usual, the following may not be correct.

My subjective impressions and thoughts:

1. To comply with FSDG and Parabola standards, it's necessary to either convince upstream to make some changes, or to do them downstream. Examples of these changes include removing the following:
   1. recommendations of nonfree software like Firefox Developer Edition
   2. DRM implementation (please see note [1] for details)
   3. GMP (I guess [1] applies here too), telemetry-, health-, crash-, other-reporting implementations that are currently just disabled via preferences / policies.
2. Examples of the changes that are already applied upstream:
   1. most of the rebranding changes
   2. most of the URLs to Mozilla domains removed via preference overriding mechanism, similarly to what Parabola does in vendor.js
   3. some functionality is disabled via either preference overriding or built-in Enterprise Policies, for example:
      1. DRM
      2. GMP
      3. *-reporting
      4. Pocket
      5. Sync
      6. extension autoupdates
      7. "Recommendations" from addons.mozilla.org (AMO) for extensions and themes
      8. default search engines
3. Development appears to be active.
4. Release delays after a few recent official Firefox releases have been no more than a few days.

Benefits for Parabola:

1. Hopefully less effort for updating the package, since most of rebranding and deactivating of unwanted functionality is handled upstream.

Potential issues:

1. It's not clear if project understanding of software freedom concepts aligns with FSDG and Parabola standards. I guess it's necessary to try to communicate and sort this out with LibreWolf project first.
2. Default preferences might put off some users, as sometimes they might seem too strict. Like disabled Sync or most of WebRTC that might be useful for free software like Jitsi.
3. I couldn't find any code for langpacks in the repos.

[1] AFAIK Iceweasel has the implementation in place too, and it's just disabled by build flag that mostly switches preferences off. Although I couldn't make it to actually download Widewine CDM after flipping appropriate preferences, one can clearly see that EME related stuff is still being compiled during build. Ideally this implementation should be removed from the source tree before build, because FSDG requires the following:

For instance, a free system distribution must not contain browsers that implement EME, the browser functionality designed to load DRM modules.