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Answers:
Yes, in fact I'm doing it right now.
Yes, if you wanted to.
Yes, or you could get the Firefox plug-in that opens a new tab with the IE engine.
Yes you can do that without any issue. Infact you can open 2 browsers at the same time and go to say www.yahoo.com at the same time.
Use firefox if you want to avoid pop-ups and malicious virus which are more on IE