Yeah, the official Firefox build unfortunately depends on some proprietary libraries, which are removed by Fennec.
Fennec is just a collection of patches to remove that from the official Firefox repo: https://gitlab.com/relan/fennecbuild
By not on android I'm assuming you mean F droid. I believe the only difference is Fennec has some of the telemetry removed, mainly Google analytics, which can be turned off in the privacy settings of default Firefox anyways.
I would suggest Mull browser. It's more hardened. I use Fennec as an alternate browser for work stuff just to compartmentalize like on my computer (librewolf and firefox)