I tested in Firefox on Linux. It works well except:

1. Copy and paste does not work and this could be bad for basically every editing task (programming etc).

2. The screen freezes sometimes and keypresses don't appear. Usually pressing backspace unfreezes the screen. Firefox about:performance doesn't show any CPU usage in the tab. Linux's htop (on my machine) doesn't show anything strange with Firefox.

And a question: I saw that new files persist after closing the tab and opening it again. I didn't investigate my localstorage etc with Developer Tools. Did you use that or some other browser feature? How truly persistent are those files?