there's nothing to stop you installing both opera and firefox, for free, on linux or windows (and on macintosh and probably BSD too), but opera does have the adverts. i used to prefer opera over mozilla, but i really think firefox has overtaken it. if you also use the prefbar thing, then most of opera's functionality that i actually used is present in firefox, and because it's open source, firefox and mozilla are going to continue to surpass closed things like opera, although i do agree, opera still has some funny little things that other browsers don't have, and if you use those, you might not find it too easy to just change.
but if you use IE, there's really NO excuse.