Microsoft has released Internet Explorer 8 beta 1, but so far it doesn't seem to be the drastic overhaul that Internet Explorer underwent with version 7. That's not to say that there aren't any substantial new features, like the potential for Firefox extension importing, but this beta indicates that Microsoft is building on what they created in the last version, not reinventing their wheel.
Microsoft is promoting the Activities and Web Slices, a "web clips" feature that updates only selected parts of Web sites, such as front-page headlines or eBay auctions. What I found most interesting is that during the installation process, when you are asked if you'd like to import from another browser IE8 includes "extensions" with "bookmarks" and "feeds".
Never say never, but I think the chances of the final version of IE8 supporting Firefox XPIs is slim. What's more likely is that Microsoft is trying to build the IE community and developers into creating more add-ons for IE. In this beta, when you import extensions, IE opens up the Windows Marketplace Web site and automatically searches for analogues to your Firefox extensions.
The Activities feature is a place to consolidate things you do on the Web: blogging, searching, word definition, and so on. There's also a toolbar button that switches the rendering engine back to IE7, but it requires a browser restart and frustratingly there's not a button for that.
The default setting for IE8 is to strip out the favorites from behind their toolbar button in IE7 and give them their own toolbar, but this actually detracts from the slick interface design. As with most Microsoft products, IE8 requires a system restart.
Empirically, pages seemed to load at about the same speed or a little bit faster as compared to IE7. Compared with Firefox 3 beta 3, though, and Firefox was noticeably faster on graphics- and script-heavy pages.
Don't forget that this is a beta release, and so it's likely to be very buggy and work imperfectly at best.