Firefox doesn't style :empty elements

Question

Here is a snippet with a sample code:

table {
  border-collapse: collapse;
}

th, td {
  border: 1px solid gray;
  padding: 3px 6px;
}

[contenteditable]:empty:not(:focus)::before {
  content: attr(data-placeholder);
  color: gray;
  font-size: .9rem;
}

<table>
  <thead>
    <tr>
      <th>Firstname</th>
      <th>Lastname</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td contenteditable="true" data-placeholder="Firstname"></td>
      <td contenteditable="true" data-placeholder="Lastname"></td>
    </tr>
    <tr>
      <td>John</td>
      <td>Doe</td>
    </tr>
  </tbody>
</table>

In Chrome and Safari, it works pretty much as expected:

For some reason, in Firefox, the contenteditable tds don't get the placeholder:

How can I fix this issue?

---

EDIT: It seems this is issue is more related to :empty than [contenteditable] as this code kinda works:

[contenteditable]:not(:focus)::before {
    content: attr(data-placeholder);
    color: gray;
    font-size: .9rem;
}

But then the placeholder is always shown, hence not being an actual "placeholder" anymore.

Answer 1

As @DreamTeK mentioned, Firefox seems to add a <br> in empty contenteditable elements. His answer, using input instead of contenteditable is valid.

In case you have no choice but to use contenteditable, here is a fix in JS to remove this unwanted br:

// VanillaJS
document.addEventListener("DOMContentLoaded", () => {
    document.querySelectorAll('[contenteditable]').forEach((elt) => {
        if (elt.children.length === 1 && elt.firstChild.tagName === "BR") {
            elt.firstChild.remove();
        }
    })
});

// jQuery
/*
$(document).ready(() => {
    $('[contenteditable]').each((_, elt) => {
        if ($(elt).children().length === 1 && $(elt).has('br')) {
            $(elt).html('');
        }
    });
});
*/

table {
  border-collapse: collapse;
}

th, td {
  border: 1px solid gray;
  padding: 3px 6px;
}

[contenteditable]:empty:not(:focus)::before {
  content: attr(data-placeholder);
  color: gray;
  font-size: .9rem;
}

<table>
  <thead>
    <tr>
      <th>Firstname</th>
      <th>Lastname</th>
    </tr>
  </thead>
  <tbody>
    <tr>
      <td contenteditable="true" data-placeholder="Firstname"></td>
      <td contenteditable="true" data-placeholder="Lastname"></td>
    </tr>
    <tr>
      <td>John</td>
      <td>Doe</td>
    </tr>
  </tbody>
</table>