Editor - is this Chrome only?

I use firefox as my browser.
I noticed when opening the Editor, it just displays a dark grey screen (the content within the browser window). Nothing else - just grey. Wanted to suggest support for firefox if possible.

Hmm the Firefox has been always supported. What OS are you running on and what FF version?
Any logs in the debug console?
Perhaps try to disable plugins you have … those sometimes block some content and similar. And maybe try and run in the Incognito mode.