How would you define "accessible"? The web app I'm working on works in Firefox, but a few text labels are misaligned with their input controls due to slight CSS deviation from Chromium. It's those things that are most of the problems for supporting both browsers, functionality-wise they're very close (except newer features that Firefox hasn't implemented yet or Google-specific features like WebUSB).
I thought it was funny when cockpit a web interface for Linux servers said my Firefox browser was out of date. It locked me out for security reasons until I accessed it with an updated version of Firefox. I was using archlinux and ran updates that morning.
It wasn't that inconvenient I just SSH into my server for whatever I was doing and they fixed it in about a week.