Alright, I just took out and put back every single piece of code on my site in an attempt to find the "thing" that is causing this margin issue, and I think I've found it. Although, it makes no sense whatsoever to me. Maybe it will to someone else.
You know AddThis? You put some code on your site and it puts a thing with all of the links to digg and twitter and stumbleupon etc. etc. etc.
Well, I use AddThis on my site, and when I remove the code for it, the strange margin above my header disappears.
Bare in mind, the code for AddThis is actually NOT in my header (which is where the margin problem exists). The AddThis code is much lower down in my code.
Here's what the AddThis code looks like, by the way:
<!-- AddThis Button BEGIN -->
<!-- AddThis Button END -->
What about this code is causing a 20px-25px margin above my header 100% of the time in Safari, and maybe 1% of the time in Firefox?
Or, better yet, how can I stop it?
Thanks again for the help.