You can see what Fennec removes or patches out compared to upstream Firefox:

https://gitlab.com/relan/fennecbuild/-/blob/master/fenix-liberate.patch

https://gitlab.com/relan/fennecbuild/-/blob/master/gecko-liberate.patch

https://gitlab.com/relan/fennecbuild/-/blob/master/prebuild.sh?ref_type=heads

It should be noted, some so-called tracker detectors will find false positives for various proprietary libraries that are “stubbed out” - that is, the interfaces are still there but they have been replaced with empty functions that do nothing.

According to the tracking antifeature on Fennec, it “Connects to various Mozilla services that can track users.” SkewedZeppelin (former Mull maintainer) lists some of these in this post. I should be clear that none of these are “trackers” but they are unsolicited connections to Mozilla services that could be used to track users.

I don’t know about Ironfox, as far as I know Mull was based on Fennec F-Droid and Ironfox claims to be a continuation of that project, but I can’t tell how close Ironfox is to Fennec nowadays.