The add-on was probably ClamWin Antivirus Glue. It's not supported by Firefox 3, but it's completely unnecessary anyway if you're using the Windows (rather than Linux) version of Firefox. Downloads are automatically routed via your antivirus software, without the need for any add-ons, by Firefox 3.
Quote:
"Firefox 3 integrates elegantly with your antivirus software. When you download a file, your computer�s antivirus program automatically checks it to protect you against viruses and other malware, which could otherwise attack your computer. [available in Windows only]"
Source:
http://www.mozilla.com/en-US/firefox/features/
Chris