Do you use the FireFTP add-on? or how are you opening ftp?

Yes I am using the add on. tried re-openning Firefox several times as this had worked in the past. Since I posted the question it has worked OK again, but I would like to find out why it happens in order to stop it doing it again. Mike

Hard to say. My guess would be that a file was locked or otherwise unavailable for some reason. FireFTP uses Firefox's password database to save your username/login, but I'm not sure where it stores the server information.