I’ve been using both Firefox and Chrome since they came out and they have always felt near each others performance. It would be interesting to see some data on the subject.
One huge advantage Chrome had was it's multi-process model. This was back in the day when stuff like Flash would often nuke your entire Firefox session.
At least on Linux, I've found Chromium to be dramatically more responsive than Firefox every time I've tried switching (since about the time Chromium became usable on X11). Firefox still drops frames for me, which I find to be exceptionally jarring; and in combination with a UI that I'm not really that fond of, a backlog of basic standards compliance issues that I've been burned by as a developer (and as a user), and generally less-useful built-in fine-grained script and cookie controls, it's never really made sense for me to use Firefox.
Now I use Brave, and it's cool, it also encourages you to use interesting stuff like IPFS Companion, which I didn't know existed for Chromium until I switched to Brave.
I think even if Google ruins Chromium, it'll continue in the open somewhere else, and still be better than Firefox at the things that are important to me.
arewefastyet.com used to show benchmark comparisons between Firefox, Chrome, and Safari. It seems to now only show Firefox's performance across a slew of benchmarks. However, the old metrics are still available on the Wayback Machine:
http://web.archive.org/web/20110901000000*/arewefastyet.com