In Firefox, hitting the Enter key on a rich text editor inserts a <br> element, while Internet Explorer generates <p> and Google Chrome adds <div>. After much discussion, Firefox 60 changes the behaviour to match Chrome, so <div> will be used as the new default separator.
If you type “Firefox” and hit Enter between “Fire” and “fox”, the result HTML would be <div>Fire</div><div>fox</div> instead of Fire<br>fox.
This behaviour, already enabled by default on Firefox Nightly and early Beta/DevEdition since version 55, can be controlled using the DefaultParagraphSeparator command for the execCommand method.
// To insert <br> as before
document.execCommand("DefaultParagraphSeparator", false, "br");
// To generate <p> instead
document.execCommand("DefaultParagraphSeparator", false, "p");