I am guessing you are using uBlock Origin or similar with Fenec, as I just get "partial protection" in the EFF test unless that extension is installed.
I am using SparkOS on this device. My usual daily driver running Android 14 uses degoogled BlissROM and gets the same results, but I just borked it testing the latest alpha version for the maintainer.
The EFF is very good at what they do, and I've found it to be reliable, based on auditing traffic with tools like TrackerControl and AppManager as well. There is also resources noting test results such as https://privacytests.org
Not quite for me - Fennec (at least the build from F-droid) does not randomize fingerprint, AppManager and TrackerControl both show it has embedded trackers sending telemetry to Mozilla, and Fennec is not available on desktop. Librewolf is hardened Firefox on desktop, and is very good though, and has the trackers going back to Mozilla removed, but it does not randomize fingerprint.
Librewolf + arkenfox user.js maximum security profile will pass EFF and about every other test you could think of. The real problem is that security comes with cost to convinence. Multi session cookies and site history suck for security but are really convinent tools for browsing the modern internet.
I like Librewolf a lot, and do use it sometimes on desktop, but they do not offer a mobile version, and there have been a few web pages I have found it was not compatible with.