Thoughts
Firefox bug: in a multi-line text area, make a selection up (eg dragging up or pressing shift and up). Then press the down arrow key. In
macOS native text inputs, this should put your cursor on the line below the bottom-most line of the selection. In Firefox, your cursor ends up on the line below the top-most line of the selection.
Sublime Text is doing a wonderful job of behaving correctly in the most obscure test cases I can throw at it, so I'll probably be using it as my editor for another 10 years.