Seems to be an issue with FF's Javascript engine.

<script type="Text/Javascript">
   var mywidth  = screen.width;
   var myheight = screen.height;
   if ( screen.width=='320' && screen.height=='396' ) { 
      myheight = '480'; 
   }
   document.write(' Note: Your screen\'s resolution is <strong>'+mywidth+'x'+myheight+'</strong> pixels.');
   </script>

The plot thickens... When I reset the zoom level (View -> Zoom -> Reset) on the interfacelift.com page, it reports the correct resolution. So FF 4 definitely has some zoom issues, as I already pointed out here:

https://support.mozilla.com/en-US/questions/751916

After some further research, the Javascript properties 'screen.width' and 'screen.height' *do* depend on the zoom level. (Which makes absolutely no sense, but whatevs.)

So consider this one solved. :)

Apparently this is (what I would consider) a bug in IE 8, as well as all versions of Firefox.

http://www.readableweb.com/forIEteam/IE8zoomanalysis.htm

Screen height and width are absolutes and should NOT, in any way, depend on the zoom level.

I can confirm that Chrome does not exhibit this bizarre behavior. I should report this to Mozilla as a legitimate bug.

I have the same problem after upgrading to FF 4, the screen keeps switching to 640X480 resolution when it should be 1388X768. I am so sick and tired of having to reset the screen resolution for every page I visit. Furthermore, when I try to upload something, the strange 640x480 screen pops up and it freezes. Why this constrant change in screen resolution even after it's been changed on my control panel? WHY? This is very, very annoying!