Opera 9.10/FreeBSD, Seamonkey 1.1/FreeBSD, looks pretty good on both! I congratulate you.
I overlaid the two until the dog lined up whenever I switch between the two browsers. Only thing worth noting I thought was due to local config (text wrapped in different places), but then I noticed that the "search" button is quite a bit smaller on my Opera (being an Old Fart and running at something like 1280x1152, I prefer it bigger, heh). I didn't look at the CSS ... are you feeding us the same stylesheet regardless of user-agent?
Anyway, nice stuff.