I've enabled the New wikitext mode on Beta preferences, and it makes the "Edit source" mode unusable from Firefox on Android. The caret is shown at a different point to where the text is inserted (see images below). The same problem appears on Firefox Beta for Android.
I tried to edit article Strategies for Engineered Negligible Senescence, section Scientific controversy, and the following happens:
1) I tap in front of "Science fact..."

2) The window scrolls automatically to the top of the text area.

3) I scroll down to the paragraph I want to edit. The caret is shown where I tapped the screen (as seen in Image 1). But when I type "Example text", the text is entered here instead:

The interface has problems also when selecting text by long-pressing and dragging the finger; the drawn selection box is not vertically aligned with the text.
It is possible that the extension is interacting with some of my scripts at commons.js or commons.css for the old editor.