Firefox is also less vulnerable to attacks by "malicious code" according to my husband. He says its not perfect but it's more secure than IE and the developers are quick to put out a patch whenever a flaw is discovered.
I like the tabs and the search engine bar that will let you search google, amazon, ebay, and a few others without having to go to the website first. I think you can add more if your favorite one isn't there. And I like that you can type something like "starbucks" in the address bar, and it will usually take you to starbucks.com without having to type ".com."