I stumbled across this post while trying to sort out the same problem. Finding no other solutions out there, I then tried diving into the code, and found out the following:
In the file <your DNN Directory>\Website\controls\SolpartMenu\spmenu.js, I commented out the following line
oIFR.style.display="block";
from the following function
function spm_iFrameIndex(eMenu, bShow, sysImgPath)
(A better solution may have been to detect for IE, and skip over it if found, but my project requirement are for FF and IE only) I believe this is the last line of the function. I haven't tested it much (at all?), but so far it seems to work in both IE and Firefox. Hopefully that will at least point someone in the right direction 
Good luck.