'(org-file-apps
    (quote
      ((auto-mode . emacs)
      ("\.mm\'" . default)
      ("\.x?html?\'" . "/usr/bin/firefox %s")
      ("\.pdf\'" . default))))

(setq browse-url-generic-program
      (cond
       ((eq window-system 'mac) "open") ; mac
       ((or (eq system-type 'gnu/linux) (eq system-type 'linux)) ; linux
        (executable-find "firefox"))
       ))

emacs --version
GNU Emacs 25.1.1

(setq org-file-apps
    (quote
        ((auto-mode . emacs)
        ("\.mm\'" . default)
        ("\.x?html?\'" . "/usr/bin/firefox %s")
        ("\.pdf\'" . default))))