It appeared as just as I was shutting down the computer and the computer said that firefox was still running at the time (or possibly an approximation) and had to force a shutdown but the error message was still there. That said, the error message did not look like a normal firefox error message in that it was all grey with the traditional red and white X in the corner but without defined borders like any other firefox error message. I'm worried this could end up being a virus of some sort.

From what I remember the message was about the shutdown of firefox being too soon as something was still running within the firefox software for it to close down. This could be the case but I'm more worried about this being a delayed reaction from a pop-up window that opened in the past or malware/spyware disguising itself as an error message as paranoid as that may sound.