I do this all of the time since FTM uses a brain dead version of IE.
It is my understanding that the default OS system browser is the one brought up when in the search results page, you click "Open New Windows" in the "Browser Options" in the upper right of the search results box. I know that my OS default browser is Firefox, and when I click the "Open New Windows" in "Browser Options" it opens in Firefox. Note: "Open New Windows" in "Browser Options" is not the same as "Open New Windows" when you right click of one of the items found in the search.
As far as changing the default browser in FTM I don't believe it is possible because FTM is written into an older version of IE. This is why there are no tabs in the browser in FTM