Once upon a time that may have been a very hard to dispute thing, but I'm not so sure anymore.
For starters, recent Firefox versions do proper process isolation per domain, similar to Chrome, which has long been claimed to be the primary security benefit of Chrome-based browsers.
In addition, the Firefox codebase is partially rewritten in Rust. This effort is incomplete and likely always will be, but it's still significant. Keep in mind that according to Google, 70% of security vulnerabilities in Chrome are related to memory unsafety!
For starters, recent Firefox versions do proper process isolation per domain, similar to Chrome, which has long been claimed to be the primary security benefit of Chrome-based browsers.
In addition, the Firefox codebase is partially rewritten in Rust. This effort is incomplete and likely always will be, but it's still significant. Keep in mind that according to Google, 70% of security vulnerabilities in Chrome are related to memory unsafety!