I still use IE for work. For some things IE is still needed, but for most things I have not had any trouble with Firefox and have found my browsing experiences so much better. There are so many great add ons for Firefox and the virus writers do not target it (yet). Since I have been using it (prob close to a year now) I have had maybe 6 spyware programs install on my system. Before that I has hundreds upon hundreds and every time I cleared those out more would just take their place.
That extension I sent you Sandyj is great. One of the things it lets you do is set it up so that if there is a site you use that you know you will always need to open in IE then you can tell it that and it will do it automatically. Otherwise you just go to a site and if you find it does not work then you right click and select open in IE and it will launch a new IE window with that URL.