You didn't get a whole lot of responses, but honestly, I'd bet money that you would get even less, if you posed the question the other way around. Because how is Chrome/-ium better than Firefox?

To give some rough image from what I know about the browsers and what I've heard other people say:

Performance: Roughly equal, Chrome seems to still be more consistently fast, which Mozilla is still cleaning up after that big architecture change. Mozilla also has more in the pipeline, which I'm not seeing as much from Chrome.

RAM use: Firefox is still considerably lower here, even though the Quantum iteration needs more RAM. There's little motivation for Google to have users use software outside of their web browser, they can't display ads or gather data in those, so there's little motivation for them to not eat up all of the RAM.

Customizability: Clear win for Firefox. You can drag UI elements everywhere you want, color the whole UI with extensions as you like or even fuck around with CSS to alter its look.

Extensibility: Chrome still has more extensions in numbers, mainly because Mozilla does not allow telemetry in add-ons (unless the users opts in).
Firefox extensions are more capable, though.

And Chrome's extension store is a dumpster fire, filled with malware. Mozilla vets extensions with actual human beings, which Google doesn't consider an option.

Security against script-kiddies: Also roughly equal. Chrome has to a minor degree still a more secure architecture (sandboxes each tab individually most of the time, whereas Firefox sandboxes them in groups of how many cores your CPU has, for performance reasons), but Chrome on the other hand has some glaring idiocies here and there.

For example Chrome's autofill fills in data in all input fields on the page at once, meaning that it will also fill input fields that you as a user can't see, so you might send off your address to a sketchy site without knowing about it. Another example is them shipping the WebUSB-API in a form that made Yubikey Neos completely exploitable, as webpages could literally just connect to the Yubikey on their own and read out the secret, bypassing the U2F API that Google had built into Chrome.

Security against Google and in extension US intelligence agencies (and in extension non-US intelligence agencies): Well, you can probably guess by yourself. Chrome Sync by default uploads your browsing history and such to Google's server in decryptable form. So, Google can access it, and because of US law, the US intelligence agencies can grab it from Google's servers, too. And because those intelligence agencies are friends with other intelligence agencies (Five Eyes etc.), those likely have your browsing history, too. Obviously depends how much you consider these a threat, but it's certainly not in your favor for these groups to have your data.

You can bypass that, by enabling end-to-end-encryption, which Google requires a second password for, so it's not necessarily an argument when you know about it, but that brings us to what the article mentions, too.

Defaults: Firefox Sync is end-to-end-encrypted by default. Only one password needed. Firefox's Private Browsing mode ships with Tracking Protection, no (potential malware) extension needed to block trackers, not that Chrome even allows extension to run in Incognito mode.

And these are just superficial examples. We're talking about millions of lines of code, tens of thousands of design decisions. In one case made by a non-profit, that always tries to protect users while trying to not piss off webpage owners too much, in the other case made by a company that always tries to satisfy its own needs in the hope that users don't notice or don't complain too much. And again, millions of lines of code. Lots of shit goes under the radar that no journalist reports about. Even in Chromium.

This is for example a project that tries to fix Chromium and it admits that it's an uphill battle: https://github.com/Eloston/ungoogled-chromium