Librewolf [1] seems to be fairly active (last commit on Codeberg was last week) and up-to-date with the latest upstream releases (mirrors FF's versioning scheme and matches their latest). Has a nice focus on privacy and no-telemetry.
Floorp [2] also looks active (last commit last week), also claims focus on privacy. Based off FF's extended support releases so it may lag behind in latest features.
Waterfox [3] is also active (last commit a few hours ago), also big focus on privacy, but it uses a custom versioning scheme so I can't tell how closely it follows FF's releases.
GNU IceCat's [4] latest release was in November 02023, so it looks like it may be abandoned.
Does anyone have any experience with any of these, good or bad? With all of them more or less promising the same things it's hard to tell which one may be the better option.