I currently use Firefox for all my browsing needs. I still find times where I have to use IE as the browser (I.E. 7). Our time entry system at the office needs IE to allow for text entry. I have also noticed that one of the online applications I use requires IE for it’s navigation. Other than that, I am always in firefox. One of the reasons I like Firefox is the additional extensions that are available. Weather, Developers tools, and server type.
I have found that there are some tools also as valuable for I.E. – Developer Toolbar Beta – (http://www.microsoft.com/downloads/details.aspx?FamilyID=E59C3964-672D-4511-BB3E-2D5E1DB91038&displaylang=en)