Okay, first I am not expert. I have been designing for just over 1 year with DW, however. I have had the same problems, but not with frames. Creating a rule in using CSS solved my issue.
My concern would not be Mozzila, but IE. Mozzila is capitalizing on the browser market, but the majority of folks use (not me) I.E.
But, have you tried using margin: 0px;? Furthermore, why frames?
I'm sure a web guru will have a different opinion, and I'm willing to learn the alternative too.