Apparently, there is some new (well, new to the mac at least!) malware that takes over your clipboard. There appears to be javascript code on some popular sites that keeps overwriting the contents of the clipboard with a malicious URL.

Reports of a solution vary between closing your webbrowser and completely rebooting your mac. A sugestion: REALLY close your browser. That is: don’t close all windows, MacOS will still have your browser open if you just close all windows. Choose Safari -> Quit Safari and/or Firefox -> Quit to really close the application, that should fix the issue.