Every OS comes with a browser other than firefox by default (except certain flavours of Linux, but they could make the choice part of the install process). If the user needs to look at something on the internet right now, they won't go searching for firefox, downloading and installing it. Having one simple checkbox that takes half a minute to decide on won't make installs take all that longer. Or just display a configuration page the first time the browser starts up with a "you can set all these later too from <here>, btw" message.