I can confirm this. Firefox 42.0.1 on Lollipop (Android 5).
Upon pressing enter after entering some text, cursor goes to next line and then jumps back to previous line.
In addition, if you tap the screen to move the cursor down after pressing enter a few times, the cursor does go to the spot where you tapped. However, as soon as you start typing, the cursor jumps back to the last line with text.