This is a great question, and it gets to the heart of practical privacy online.
DeGoogled Chromium does actually have less telemetry problems than Firefox, so it's really easy for DeGoogled Chromium proponents to say that it's the most private. The issue is that DeGoogled Chromium is Chromium, and Chromium is a less privacy-capable browser engine than Firefox.
That could be a longer conversation, but the short versions:
- Chromium lacks a number of privacy features that Firefox has, including some anti-fingerprinting options that can be enabled through `about:config`, and container support, which is a really big deal for isolating site data and avoiding correlating user sessions on websites like Github/Youtube/etc... with incidental visits to those sites.
- Chromium's extension API is hobbled, particularly in a couple of areas that Ublock Origin cares about. The wiki goes into more detail on this[0].
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The mistake is in looking at the small amount of (admittedly bad) data-leakage that Firefox does have and being so worried about that information being sent to Google/Cloudflare that you pick a browser that is less good at keeping you private on every other site you visit, including visits to Google/Cloudflare pages.
Thinking practically about this stuff is just a really hard thing to learn to do, at least it is for me. Maybe other people are magically good at it. But I regularly find that it's helpful for me to sit down and think through my privacy goals more tangibly in the form of "how much data is X actually leaking, what should my priorities be based on the volume/nature?" A lot of people worry about privacy problems in the wrong order.
DeGoogled Chromium does have better defaults than Firefox in multiple areas. It's just that the privacy benefits from those changes don't outweigh a crippled Ublock Origin install.
I'm curious about why you believe this about Ungoogled Chromium, as I had concluded the opposite after researching.