I will start from the bottom and work my way up.
Importing - This means to copy all of your favourites, bookmarks and history (I think the history is moved too, but it doesn't matter) to another browser. So if you did wish to use firefox you could have all your favourites and bookmarks that are currently on IE.
Firefox, I believe (I use firefox) is alot fast and less buggier than Internet Explorer, but, people may say different. Theres no harm in trying it!
It is simple to go back on internet explorer, you just re-install it (internet explorer can never actually be deleted as it is a part of the Microsoft operating system so getting it back isn't too hard to do).
I hope that has answered your questions.
Diz,