After updating to Firefox 18.0.1, I am getting a "User Registration" page. What is this?

When I open Mozilla, I get two windows. The one on top is identified as "User Registration"; it is blank except for the line, "%brandDTD; %browserDTD; %baseMenuDTD; %charsetDTD; %textcontextDTD; %customizeToolbarDTD; %placesDTD; %safebrowsingDTD; %aboutHomeDTD; %debuggerDTD; ]>", at the top. If I close that page, my usual home page is right behind it. It is apparently causing no actual problems, but is disconcerting. What is this page, is it asking me to do something (if so, what), and, if not, is there a way to stop it from appearing?

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