User mentions copied within the editor are corrupted in firefox

Environment:

Confluence 6.12 (Linux 64bit), postgres9.5
Client machine:
Windows 10,
Firefox 62.0.3 (64bit)

Bug description

User mentions copied within the editor are corrupted (firefox 62.0.3)

Steps to replicate

1. Create new Confluence page, add user mention and save the page
2. Open edit page.
3. Select existing user mention and copy it to other section of the same page.
4. Save the page

Expected results

Page contains two user mentions

Actual results

• Copied user-mention  is visible in the editor but it is not visible after saving the page 
  First user mention is saved in page storage format as:

  <p>
    <ac:link>
      <ri:user ri:userkey="ff80818166627e6a0166628149a90000"/>
    </ac:link>
  </p>
  

  (correct form)

• Copied user mention is saved as:

  <p>
    <ac:link/>
  </p>