Harry was very upset about something, and wanted a chat with Sirius. Unfortunately, all our communications with the outside were monitored, with the sole exception of Umbridge's own fireplace. Ginny Weasley came up with a way for Harry to use that particular Floo connection, arranging for Fred and George to cause a distraction. I wasn't too sure about this myself, remembering how well her little brainstorm about "Dumbledore's Army" had worked, but Harry wanted to go through with it, and I couldn't stop him without sneaking to Umbridge, which I'd rather have died than do.