So I added editor.use_div_for_default_newlines back to about:config.
Line 1
Line 2
Line 3 (newline follows)
Line 4
Wait a minute - how are you getting the Line 1-3 following each other with no added line space? Maybe I'm misinterpreting what you meant by "newline"?
In the following, I'll use
<Enter key hit here> to signify I actually hit the 'Enter' key. If I'm referring to the 'Enter' key, w/o actually hitting it, I'll just say 'Enter':
-----start after this line-----
I write this and then I hit 'Enter', and I'll be on the next line
<Enter key hit here>
And now I'm here - no extra line space between the lines, like your Line 1-2-3
<Enter key hit here>
<Enter key hit here>
And that was two 'Enters'
<Enter key hit here>
<Enter key hit here>
--------------------end--------------
So it would look like this:
I write this and then I hit 'Enter', and I'll be on the next line.
And now I'm here - no extra line space between the lines, like your Line 1-2-3.
And that was two 'Enters'
And now I'm done - ERD50