Put a gentle "Use Firefox" (or any other non-Chromium-based browser) message on your website. It doesn't have to be in-your-face, just something small.
I've taken my own advice and added it to my own website: https://geeklaunch.io/
(It only appears on Chromium-based browsers.)
We can slowly turn the tide, little by little.
Small anecdote: I am not sure how you're detecting the browser, but this note still appears in Orion (webkit-based browser) while it does not in Safari. Persists even when I change user agent explicitly to Firefox or Safari.