The code that auto-expands the editor textarea as you type appears to render the editor unusable on Firefox (T214880).
As a temporary fix, let's disable the auto-expand feature in Firefox.
- Detect the browser is Firefox
- If Firefox, do not change the height of the textarea
- Escape the resizeEditor function when the browser is Firefox
- Open editor in https://en.m.wikipedia.beta.wmflabs.org/wiki/Spain in Firefox.
- Confirm the textarea has scroll
- Open editor in https://en.m.wikipedia.beta.wmflabs.org/wiki/Spain in Chrome
- Confirm the editor in textarea takes up full screen