In mine it means Junk, but the icons vary between platforms and themes. In some other places it could mean delete.

Would you care to post a screenshot so we can see where it appears?

https://support.mozilla.org/en-US/kb/how-do-i-create-screenshot-my-problem

Strange - I went to my email listing to get the screenshot you requested and the message was gone! Could the icon mean that the message was being recalled by the sender? Interestingly, I went to my email forwarding service (which forwards to TBird) and the message was still there and I was able to open it.

I guess we'll have to wait for another one.

Normal email protocols don't allow for a recall. It can be done within a closed system where all the users are inside a local networking system and there is an in-house server. So the Outlook desktop client working in conjunction with an Exchange server will offer a recall facility, but it's effective only for correspondents using Outlook and connected directly to that same server using proprietary protocols.

Once the message has left the premises en route to another server it has gone, irretrievably. The best we can do with regular email is to send a second message asking the recipient to disregard the first one.