I currently use LanguageTool, although not only did they recently switch to reCAPTCHA, they've been using Doubleclick for longer than I can remember.
As such, I'd appreciate it if there were a Proton Scribe browser extension, especially because I've been planning on upgrading to Proton Unlimited, and it would seem more worth-it if there were a writing system bundled in.
I suggest making it Firefox-first to respect users who have gone out of their way to switch away from Chromium.