I use Firefox 95% of the time and Internet Explorer when I am forced to. There are some Microsoft sites that still won't allow you to use unless you are using Internet Explorer.
I have another browser and Office 2007 issue, as well. When I click a link in Outlook 2007 (on XP Pro SP2) I get an error pop-up that exclaims, "Windows was unable to locate the file." Meanwhile Firefox loads the linked page. Going back to Outlook I still have to click the OK to close the error box. I wonder how many programmers it took to make sure that Firefox users would get an error and Internet Explorer users wouldn't.