What is this error about?

I got the following error when tried to log into my site using Firefox.

{ "status": true, "updaters": [ ], "debug": [ ], "messages_error": [ ], "messages_status": [ { "id": 0, "value": "Login successful." }, { "id": 1, "value": "You have \x3ca href=\"/messages\"\x3e\x3cem\x3e2 unread messages\x3c/em\x3e\x3c/a\x3e." } ], "messages_warning": [ ], "redirect": "http://expertscolumn.com/users/nikmahajan?1276896593=1", "preview": null, "form_id": "user_login", "options": { "enabled": true } }

The error showed in a small scrollable box at the top left on the blank page and disappeared on refresh. Please help.