the SlackBuild provided with slackware 14.0 is for an older version of the application, so just updating the sources is not enough: latest
mozilla-firefox.Slackbuild in -current, beside other modifications, has a specific switch ("--disable-pulseaudio") that is needed for building latest versions of the application on Slackware (that, as Alien Bob is saying, does not provide pulseaudio).
if just adding the switch to the build script you are using is not enough, you can try to use latest script in -current but, AFAIK, it's untested on 14.0.