I don't know about the latest IEs, but firefox has add-ons, that can do all sorts of stuff, for example:
DownThemAll is a download manager, with resumption and all. Since I've started using it, I've never needed to look at another manager.
Adblock Plus is an ad-blocker, which allows subscription to maintained lists of advertisers, plus manual addition. It can also be used to block images, like sigs you may not appreciate, etc.
There's the tabbed browsing, which I gather was in the latest IE. Middle-click on a link to open it in a new tab, and middle-click on a tab to close it.
Live bookmarks, which I think you already have one at install, for BBC news? Basically, a drop-down list of links, taken from an RSS feed.
There's also the search box in the upper right (which may have been in IE, I don't know). You can add search engines to it, which you can then select to use, and perform searches with the selected one, by typing the search in that box. It doesn't sound that useful, but I couldn't live without it.
A downside would be that it adheres to the W3C standards, moreso than IE. So pages that may have looked fine in IE might not in Firefox, due ti IE creatively interpretting the code.
Plus, there are some sites that have functionality that flat-out won't work in Firefox. But that's thankfully rare.