Comment 20 for bug 240736

I run 1024x768 and have exactly this issue. After playing around in localstore.rdf I found that these lines should be changed:

  <RDF:Description RDF:about="chrome://browser/content/browser.xul#main-window"
                   screenX="0"
                   screenY="0"
                   width="1024"
                   height="768"
                   sizemode="maximized" />

Into these:

  <RDF:Description RDF:about="chrome://browser/content/browser.xul#main-window"
                   width="800"
                   height="600"
                   screenX="4"
                   screenY="139"
                   sizemode="maximized" />

Which somehow fixed it. Apparently setting the width+height to a value too high (for instance the exact screen resolution) indirectly causes the issue. I don't know what the real problem is.