It’s not in the official repos, but it’s really simple to add the LibreWolf repo and use that.

Other than that, there’s not really much in the way of Firefox forks. I believe the Debian builds have their own configurations as well, but I’m not certain.

Off-topic, LibreWolf uses the extrepo package to add their repo which is a great third party repo management program for Debian. It has a curated (by members of the Debian packagers) selection of other third party repos for some popular software that either doesn’t make it into the official repos for whatever reason or aren’t kept super updated in Debian Stable.