What kind of CPU do you guys have? I realize I'm coming from a different vantage point, PC, but I think Firefox does demand a certain threshold for processing power.
I usually have 3 windows (with 7-8 tabs in each) open at once, and I've never slowed down. Not sure how OSX handles memory, but if you notice even a slight slowdown, in Windows, if you minimize it XP will redistribute the memory Firefox is using--"refreshing" it if you will.
I dunno how OSX will do it, but aside from those 2 factors (memory management and CPU power), i'm not sure what else would cause Firefox to slow down. If i had the same # of IE windows as tabs in FF (21-24 windows), I'd be taking over 200MB+ in memory there >> and that is far worse than the 40-50MBs Firefox can take.