Thanks for the clarification! This is what I suspected, too. What matters to me is that it works with custom extension lists. I am not sure I understood what was special about the original "2020 Fennec".
There's also a note from someone on Reddit [1]:
> Fennec is also F-Droid's firefox version. It differs from official ones by having proprietary bits and all telemetry removed as well as unlocked about:config at the moment.
In the description on the F-Droid page it says "Fennec F-Droid is based on the latest Firefox release (codenamed Fenix).".