Unlinking mentioned user does not remove mention formating in Firefox

Summary

While using Firefox you're trying to remove user mentioning link using "Unlink button" the mentioning formatting stays in the storage format.

Environment

Any supported client configuration with Firefox.

Steps to Reproduce

1. Create a new page
2. Add a User Mention
3. Save the page
4. Edit the page
5. Click on the user mention and remove the link using "Unlink" button
6. Save the page. Examining the page source code you can see that:
   <li class="confluence-link user-mention current-user-mention">
   Edit the page
7. Mark the no longer linked Full Username with the keyboard (cursor keys) or by using the mouse (but NOT by hitting the left mouse button three times, thus marking the whole line) and overwrite the user name by some text, eg. "Hello World"
8. Save the page. "Hello World" is rendered as a user mention.

Expected Results

The link and the formatting of the user mention are removed.
The storage format:

<p>Username</p>
<p>ewrewre</p>
<p>ewrewr</p>
<p>ewr</p>

Actual Results

The link is removed but user mention formating still in place
The storage format:

<p><span class="confluence-link confluence-userlink user-mention">Username</span></p>
<p>dsfsdff</p>
<p>dsfsdfdsdfds</p>
<p>fdsfsdds</p>

Notes

The bug seems to be Firefox only regression of CONFSERVER-37328