Thanks for the help.
I tried Shift-Ctrl-Del, which brought up a dialogue window offering me the choice of clearing cookies. I didn't know if this would automatically clear all cookies for all sites that I visit, and didn't want to take the chance (I like for my username, password, etc., to be remembered by the different sites).
So ... instead I went to my Firefox Tools menu, clicked "Options," clicked the "Cookies" tab, clicked "View cookies," found the one for Dreamhost, removed just it, and then refreshed my browser window. Voila! The Dreamhost panel log-in screen.
I haven't logged in yet, but assume it will work OK now ... now that I'm getting an actual screen.