Supporting the Chromium monopoly is a valid point, but there’s also a reason why a lot of browser companies, even those who market their browser at more privacy-conscious individuals such as Vivaldi and Brave, choose to fork Chromium over Firefox/Gecko. A good portion of that reason is Chromium’s superior security architecture that is a lot more battle tested and mature; the rest of the reason often comes down to compatibility and mobile-readiness.
A lot of people are wary of any browser engine attaining a monopoly since IE achieved this back in the day. It’s not exactly a like-for-like comparison though, since Chromium is actually open source and IE/Trident was not. For that reason, anything problematic can be stripped out by those who fork it which is exactly why we have browsers like Brave, Vivaldi, Ungoogled Chromium, and others who remove anything that feeds data to Google from their releases. The option also theoretically exists to hard fork the project entirely and take it in a different direction which was never a possibility in the IE days, although that would be a monumental effort.
I get it if people want to support Firefox/Gecko for philosophical reasons. In an ideal world there would be several projects of equal maturity to the browser engines we have today. Realistically though, for all intents and purposes, the vast majority of the world is already using Chromium in some flavor or another and it’s a project that has a lot of the world’s best browser engine developers contributing to it. As a user, I care most about using a secure, privacy-respecting browser that I find innovative which caters to my needs through its features rather than fighting a philosophical battle that’s already been lost. Naturally if you find Firefox does cater to all your needs though, more power to you.