Yesterday I noticed that a previously unknown icon/thumbnail named "Untitled window" had appeared in the bottom panel (besides the Menu button). When I click it, nothing happens (it doesn't open up anything) and it does not respond if I right-click it and choose "Close". I can only make it disappear by logging out and in again. I have no idea which program could cause this.
Also, it only seems to appear in one workspace - if I switch to another workspace, it is not there.
Any ideas? It is just a cosmetic problem really, but an annoying one. It appeared after the system updated Firefox 13 to a newer version 14.
I use Linux Mint 13 64-bit with MATE desktop.