FutureFox

Multi-column bookmark capability.

a RSS flock-like feature

a real-time theme changer (with no need of restart the nav)

Ability to mess around with all the toolbars without using extensions.... also themes that are all multi-compatrible.

AutoHiden funtion
everything(button, toolbar, tab etc.) hiden/collapse unless move my mouse on it.

Less memory usage.

if possiable , no memory usage :p
i don't know details about net tech things, but we have net speed highway , why still need memory. Just forget it~

making use of widescreen real estate,
it would be cool if you can actually view 2 webpages in one window (side-by-side)

PS: yes I know I can just open two windows, but what's the fun in that?

@Aphaits: I could swear I saw an extension for that. Might have been vertically aligned though.

dockable fav/history (that appear only at mouseover)... or is it already possible?

@nosphere: I'm pretty sure ez sidebar will do it.

https://addons.mozilla.org/en-US/firefox/addon/1930

Ability to display websites in thumbnails, as in IE7.

@ sidebar: just download the 'All-in-One sidebar' pretty handy, only the mouse-over thing doesnt exist yet