The built in popup blocker and adblocker extension is worth the price of admission alone. 
(I don't know anything about Spy Sweeper).
I just purchased Spy Sweeper recently after a nasty spyware problem that even spybot and adaware couldn't fix. I have Spy Sweeper configured now to guard my startup programs, homepage, etc, But I don't know enough about Firefox to switch. I downloaded it and used it a couple times, works ok, but my Spysweeper is already configured for IE and I don't know if I have to buy a new license for Firefox.
I have Free Republic, Drudge, Accuweather (with my local forecast), Google News and a couple other sites automatically load as my home page.
The built in popup blocker and adblocker extension is worth the price of admission alone.
(I don't know anything about Spy Sweeper).
I run Firefox on my Mac, no need to worry about spyware. :-)
Your Spy Sweeper license should be for you computer, regardless on the number of browsers you run. You can run both IE and Firefox.
You should also be able to add Firefox to Spy Sweeper's venue. [I don't use Spy Sweeper, so I can't tell you how. But check your options or preferences. Also check the Spy Sweeper help file.]
I might have misread your question.
http://www.webroot.com/products/spysweeper-indepth/?WRSID=b06bf5823a88e01db02d6cb24d4443f0
Down at the very bottom it says it supports IE. If it's a program worth it's salt, it will find the critters that got onto the hard drive anyway.
I recommend Spyware Blaster:
http://www.javacoolsoftware.com/spywareblaster.html
It works with IE and Mozilla, and it's free, though donations are appreciated. Remember to check for updates weekly.
Firefox comes out of the box with certain defaults that IE lacks, such as the ability to block certain downloads, "drive-bys", for instance. It's not perfect, since nothing is, but there are those who have noted an almost instant reduction in certain problems. Give it another try.
Aye!
Doesn't matter since Firefox isn't anywhere near as vulnerable as IE is to homepage hijacks, secret malware installations, and phishing schemes. That's why everyone is dumping IE because they don't have to fool with a third party app to provide security to the browser.
"Is there any Freepers out there who run Firefox and Spy Sweeper?"
If you run Firefox, you won't need Spy Sweeper. I have had exactly zero spyware items since installing Firefox in December.
Start using it, and within a week or so, you'll be used to its functions. I have been using it for just over a year, and love it.
Spy Sweeper is a bit of a resource hog - right now it is occupying 40 MBytes of my RAM. I use it on systems where I am more concerned with security than maximum performance. My PC that has my financials runs Spy Sweeper. My gaming PC's do not run Spy Sweeper.
Spy Sweeper has specific hooks that it can put in Internet Explorer to protect against having your Home Page, Search Page or Favorites modified without your consent. It does not have similar hooks for Firefox. But (a) these hooks are only a small part of the value of Spy Sweeper, and (b) so far I've never seen a need for such protection hooks in Firefox.
You won't need a new Spy Sweeper license, and your security will increase even more by switching to Firefox. Go for it.
On all of our computers here at home (and we have a bunch), I have Firefox as the browser, plus Ad Aware SE and Spybot Search & Destroy (both free; available at www.download.com ) on each. Also running Zone Alarm Pro on every one as well (PLUS Symantec AV). Firefox has been a HUGE help.
Speaking the obvious.......but products like Ad Aware, Spybot, antivirus software, etc.....are useless unless you keep them CURRENT. They can only protect you from what they "know" exists. Too many seem to forget that. :)
If you go to their site (www.webroot.com) you'll see the particulars. I love their Window Washer and Spy Sweeper, and I use Firefox. They're in Boulder, CO.