This is a nice little gem I found while perusing ComputerZen. 

I've been out-of-blog (similar to out-of-touch but with blogs) for almost a week now.  While getting back into the swing of things, I went to one of my favorite tech blogs, ComputerZen.com, and noticed Scotts entry on a FireFox extension that embeds an IE window within a FireFox tab via the context menu. 

To things I've noticed within the first 5 minutes of using:

• Tabbing around a form doesn't seem to work, and
• Typing in a url in a Tab that is already IE embedded reverts that tab back to FireFox.

Both minor in my opinion but worth noting.

Other than that, I, being a web developer, look forward to using this.  Thanks to PCMan (Hong Jen Yee) for creating this.

Down load it here from Mozilla.
https://addons.mozilla.org/extensions/moreinfo.php?application=firefox&id=1419

If you're using Firefox 1.0.x, you have to install an additional package from the following link to make it work. http://downloads.mozdev.org/ietab/ietab_plugin-1.0.3.xpi

UPDATE: For those of us who use IE and Mozilla for web designing & layout purposes, this isn't 100%.  For the most part, all is good but <fieldsets />; don't render properly (see this site: www.bit-shift.com, in IE and IE embedded - you'll notice a difference.) and if that is the case, there might be other tags/css that don't render properly.  My suggestion for web developers; keep using the actual browsers for referencing layout.