Well Chrome(ium) has almost all of the browser market share and google is trying to push something called web environment integrity which would implement a sort of certification system where web servers evaluate the authenticity of the client. If you extrapolate that idea a bit further it boils down to "we won't serve you content if we don't like your browser, device, OS, etc". Which I would consider as hostile to the open but rapidly closing internet as we know it.

Edit: I forgot to make my point lol. Firefox is a completely different browser engine from the chromium based browsers which is why you see a lot of people recommending firefox because they don't comply with web integrity. I don't think it's working though because this is something only the techbros and the cybersisters care about while everyone else just goes about their day.

Chromium is still controlled by Google, so having an overwhelming market share of Chromium-based browsers reduces competition and increases Google's control of the market's position and future. Using Firefox (and Safari, if it were not locked to a single ecosystem) reduces that threat.

It's not a stupid question, some people just don't know.

Mainly it's because:

1. Chromium holds too much market share which is bad for the health of the Web.
2. Chromium is controlled by Google which is concerning because they have been known to plant trackers even in software that shouldn't have them.
3. Chromium is inherently less secure, it contains features that might seem nice but are extremely risk to give access to websites i.e. letting websites access Bluetooth.

There are probably plenty more reasons but these are the big ones, and of coarse this is a simplification, in reality things are always a bit more complicated.