Hello ,
(please excuse my average english)

I usually use "Conditionnel Comments" to Correct IE , most of the times it's good enough to have no troubles with FF , opera and safari (for Windows ) ,now you even have chrome to test , unless the MAC Safari having difference is an old version .
(Konqueror is sometimes surprising ).
Opera and IE7 have a couple of bugs that seems to be similar (close to haslayout behaviors , zoom:1 for IE , and position:relative ; for opera can help , use only if needed of course)
Else , tag:after {content'';} gives some very good results in opera , and i found out lately in Safari too , it forces sort of a reflow . To check if this would work , just go hover a link and see if it sets yor "nav" at the right position or size expected.
... these are just some hints , but they help to find fast a solution or reason if your css is already well-thought .

has your "nav" problem got to do with margin , does it dessappear if you set margin to zero ? ...

Does it have to do with the fonts used , resolution of the screen , If both macFF & mac Safari behave the same .... then a js checking the OS would maybe more efficient to serve extra rules ?

GC

Thank you for your help...it's interesting, on tow people's Macs there is this slight glitch and on another no glitch...same browser/versions. And on http://www.browsrcamp.com/ it's glitch-free.

Again, thanks

thank you...I think that that's a good thought but I also think that the reality is at this point in time that with all the various browsers, browser versions, and that in combination with various os's, etc, that the idea of one uniform system of coding is not a possibility at this point in time. Also, a matter of development time in terms of how much one would want to search for the perfect coding in order, for example in my situation here, to code for around 4-5 pixels. In this situation I can live with a hack or two. But I certainly understand what you're saying and I agree that correct coding certainly fixes most issues.

Thanks...