Hey mtnman, look into two add-ons called AdBlockPlus and NoScript. AdBlockPlus "subscribes" to an online list of known ads and won't let them display on pages. I haven't seen more than ten ads in the years I've been using it. NoScript will greatly enhance your security but increase your workload a little bit more. When you go to a website that shows more than text and pictures, you have to allow the website to run them. It'll remember which ones are "trusted" and will keep the sneaky hidden ones from doing things with your computer.
Although, you've gotten rid of the chance of getting most of the viruses out there by dropping the ActiveX that comes with IE (it's a virus superhighway). Keep an eye on your add-ons list under the Tools menu for something called "Microsoft .NET Framework Assistant." It apparently enables ActiveX through FireFox and should be disabled.
One last one before I forget. I use an add-on called IE Tab for those very few websites that won't work in FireFox. Basically, it opens up IE inside the FireFox tab for any website you toggle it for. Comes in handy every once in a while.