And what you can and cannot do once you have one.

While it's not illegal to have a monopoly in one or more markets, it's highly not legal to use that monopoly to leverage your entry or marketshare in another.

In this case, MSIE having a default search option of MSN is leverage the OS/Browser monopoly in order to increase marketshare in the search market.

Google and Firefox are not at all in the same position (Firefox doesn't own Google, and doesn't have any kind of monopoly).

Just saying "it's configurable" is no defence.