shadow-dom example features backwards text typing in Firefox

Description The cursor is being placed at the beginning of the text area after every character typed.

Recording Firefox:

Sandbox https://www.slatejs.org/examples/shadow-dom

Steps To reproduce the behavior:

1. Go to https://www.slatejs.org/examples/shadow-dom on Firefox
2. Click anywhere in the editor
3. Type anything

Expectation Chrome:

Environment

• Slate Version: 0.63.0
• Operating System: Windows 10
• Browser: Firefox 88.0.1