Don't go back to IE ever again! Just keep using Firefox. It's much safer, especially if you use the NoScript add-on (which basically makes it so that websites can't execute scripts without your permission... so you can't get a trojan horse virus simply by visiting a website, like you could with IE. You need to be a bit tech-savvy to really use it though). The Ad-Block Plus add-on also makes it so you'll never see an online ad again (a small handful get through, but it blocks 99% of ads and unnecessary pop-ups).
I have no idea if IE has these kinds of security features anymore, but Firefox had them first. It's just a much better browser, and virtually all people support Firefox when they build websites now since it's so popular. I wouldn't go back to IE if I were you.