Note that I'm using ZAV. A while ago I mentioned memory use here (still using XP SP3).
At the time there was 645MB in use. I currently have a lot more open and only 574MB is in use. Looking at task manager, there's only vsmon.exe and zlclient.exe, with ZAV 8, you had double scanningprocess.exe entries as well, with fluctuating memory use.
Same as then, I just closed ZAV. It went from 573 to 470. So roughly 1/3 of 8's memory use. Which is as expected. I have not compared scan speeds for example, as I usually do them when I'm away and really don't care how long it takes, as long as it's not days... But I find 9 a big improvement in the performance department and overall. I've been using some kind of ZA incarnation since 4.5 and 9 has given me the least amount of issues and works best.
Don't know about FF. Seems like a logical slowing down factor.