Brave used Firefox's core Gecko (and forked it) several years ago, encountered problems with it and called it a day. Then they switched to Chromium and they are more alive than ever.
Had they kept using it, well that would have certainly been another dead Firefox-based browser fork.
Careful. Every time I've mentioned Brave here I've been downvoted. Getting on point: I've used Brave for awhile now and it's been fine. Major issues for me are updates are frequent enough to be annoying, and I haven't been able to access sites with self-signed certificates.
Had they kept using it, well that would have certainly been another dead Firefox-based browser fork.