I'm using Firefox 80.0.1 (64-bit).
Steps to repeat:
1. Find a thread and post a new comment in it.
2. Edit the comment shortly after posting it.
3. Edit it yet again.
When I click on Save Comment after the second edit, the whole comment goes up in smoke and in its place is the usual text box for typing a comment. As you can see from my comments in the linked threads above, the lost comment seems to restore itself a while later, but I'm not sure how long that takes. I'll try and actually repeat this in the next comment in this thread to see if I can grab some screenshots.
1Password Version: Not Provided
Extension Version: Not Provided
OS Version: Windows 10 1909 (18363.1016)
Sync Type: Not Provided