Browsers do not cancel each other out, so you are free to use both Firefox and IE. You could even have other browsers, and they would all work independently of each other
When you download/update firefox it gives you the option of importing your bookmarks from IE.
I've been using Firefox exclusively for years, and I always recommend it over IE. In those years (and about five computers) I've only lost my bookmarks once. And it was my own fault. And I got them back.
I can't answer your third question, but I can suggest you download Firefox and try it anyways, without dreading or hating it straight off, just for the fact that it's different than IE. If, for some reason you can't stand Firefox (I've yet to see that happen though), you can always uninstall it, and it won't effect IE at all.