readonly attribute does not prevent editing in contenteditable

Phenomenon

The readonly attribute, which should prevent editing on form inputs, does not work on contenteditable regions in Firefox. Users can still edit the content even when readonly is set.

Reproduction example

1. Create a contenteditable div with readonly attribute.
2. Try to edit the content.
3. Observe whether editing is prevented.

Observed behavior

• In Firefox on Linux, the readonly attribute does not prevent editing.
• Users can still modify content.
• The attribute is ignored.

Expected behavior

• The readonly attribute should prevent editing on contenteditable.
• Content should be view-only when readonly is set.
• Behavior should match standard input elements.