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.
The RTE was actually already supposed to be disabled for FF for Android due to a different issue (which doesn't appear to be an issue any longer), but this was being superseded by another, more general check.
I've taken out the JS involvement when creating a line break and FF on Android still seems to have issues inserting the break. It does actually do the insertion and even puts the cursor in the correct position (it's in the correct position when keyup runs), but then it seems to get moved back. In some cases, this even happens when backspacing.
It happens in multiple other RTEs as well, so I think this is an FF bug. We have re-disabled the RTE in this situation now though.