Have you tried again with Firefox recently? I'm the developer of an extension that makes extensive use of the Shadow DOM for UI components in content scripts. I

Have you tried again with Firefox recently? I'm the developer of an extension that makes extensive use of the Shadow DOM for UI components in content scripts. I recently ported our Chrome-only extension over to Firefox and had a few minor issues but none with the UI. I'm even using React for Shadow DOM UI components and it's been working well in both browsers.