Ok, the Firefox (Quantum famly, version 84) default settings seem to insert double line breaks for empty lines.
Let's try one more time, this time by toggling the Firefox's setting named "editor.use_div_for_default_newlines" via "about:config", from the default value of true, to false: "Line1" should show separated from the text paragraph that you're just before it, by exactly 1 empty line, and also "Line2" should be separated from "Line1" also by exactly 1 empty line:
Line1
Line2