I've been using firefox for a while now, but some sites just work better in IE (read SharePoint and Outlook Web).
I've used
extensions that let you open a link in IE from firefox, but thats still annoying so Ive just resorted to leaving an IE window open for the times I need it, until now.
There is a firefox extension that lets you open a page with IE as the renderer in a tab!! Its called
IE Tab and I'm converted.