NoScript in conjunction with adblock+ I find to be the worse culprits for memory usage.
Without them Firefox tops out usually under 300mb, with them I'm typically seeing 600-700mb being used. And this usage seems to grow as time goes by and my exception list grows.
That being said Firefox, much like Windows 7, releases that memory when something else asks for it. If I open Photoshop up and get it using 3gb or so Firefox will slim down to around 100mb but it also slows down as well from it.
I wouldn't be surprised if this is the case too. I run NoScript and Adblock+ with 20 tabs. They leak badly. I can't imagine Adblock being written efficiently.
FF starts around 300M. I have gigs of spare memory but once FF gets above the 800M-1GB mark, it just gets very sluggish even though I'm way below 40% total usage across all programs. I do have lots of random flash and tabs open but FF needs a restart every few days to reset back to the base 300M usage, where it's snappy again.