IE Tab
One of the key reasons I don't use Firefox as my primary browser is page compatibility. I visit a ton of sites that don't play nice with Firefox. If you want the added features of Firefox without the inconvenience of needing to constantly run in dual browser mode, IE Tab saves the day. When you need to access an IE only site, this Firefox extension launches a new tab embedding IE inside Firefox. You never leave your current browser while getting all the advantages of having both browsers running side-by-side. There are a couple of quirky things I encountered on pages that open everything in a new window, but in general, this is the best of both worlds for getting Firefox tabbed browsing and the flexibility of accessing IE only pages.