Is Fennec on Android like that? Still developed by Mozilla, but has all branding, telemetry and firefox-account stuff removed (even comes with duckduckgo as default search engine)
I think Fennec F-Droid is a straight re-compile of the official Android app with binary blobs removed. So technically it is the actual open source version. Firefox telemetry is open source (at least on the client side) so wasn't in the scope of that, but there are certainly variants that remove that as well.
Last time I tried Ungoogled Chromium, it was a pain in the ass to install add-ons, that might have changed but back then I had to download and install extensions by hand, I don't know if it's still the same process nowadays.