Does Firefox3 do this to your Oracle Apex Builder?
The fix is to disable a CSS attribute for the "fieldset.lov" class, as seen here using Firebug
To permanently fix the CSS, modify the apex_3_1.css file. Search for "fieldset.lov" and delete the "height:1em;" attribute.
I haven't tested to see if this has other undesirable side affects, but so far so good.Mark