Chromium-based browsers have strong sandboxing, and a dedicated security team at Google. If security is your main priority that makes for a compelling argument. They have strong code guidelines such as their rule of two[1], make heavy use of fuzzers and others techniques.
You can read some of the other techniques they use for security here[2][3]:
Despite these techniques, they are using a language that is not memory-safe. Security bugs do occasionally pop up in Chromium. They're also patched very quickly, but the possibility exists of a 0-day impacting users.
Firefox has improved considerably in their sandboxing, but it is still a legacy codebase. Many techniques used to ensure code safety were not originally available, but have been slowly patched into the codebase.
Firefox has one advantage: it was the progenitor of Rust. They've had a head start in building components in a memory-safe language which offers them safety that Chromium does not currently have (Google are still considering adopting Rust). For this reason it's possible that Firefox will be able to avoid a memory bug that will affect Chromium-based browsers.
Both companies offer a generous bug bounty program. Firefox is occasionally targeted as it's used by TOR users, but Chromium receives far more scrutiny overall (which isn't necessarily a bad thing).
I don't think either browser is a bad option, but neither are they perfect. I'd feel safer overall with Chrome (or Chromium, whatever) primarily due to the considerable resources Google is able to put into their security focus.
[1] https://chromium.googlesource.com/chromium/src/+/HEAD/docs/s...
[2] https://docs.google.com/document/d/e/2PACX-1vRZr-HJcYmf2Y76D...
[3] https://security.googleblog.com/2021/09/an-update-on-memory-...