Sorry I haven't weighed in on this yet, but I have a question or two. Are you regularly clearing out cookies/temp files? Are you running disk cleanup or using a cleaner like CCleaner?
Running a anti-virus alone will not protect you. There are many things that clog up your computer and slow it way down. I have run Firefox for years, and while an occasional script sneaks in, it gets stopped real quick. Here's what I run, and it's all freeware;
1. Microsoft Security Essentials Anti Virus. They update w/ Windows update and never nag you to upgrade to a "pay for version". And they have a vested interest in keeping your machine virus free.
2. CCleaner from Piroform. Used to be called CrapCleaner(so I'm told). It cleans out the temp files and cookies and history.
3. Comodo. This is a registry editor that helps virus' from changing registry values.
4. Spybot (w/ TeaTimer). This is adware protection, and among other things, will help with stray scripts.
This advice comes from a guy who goes to some nasty corners of the internet(and I'm not just talking about The Southside Dive Bar), and I've had no issues with speed.