Apart from Firefox itself having that option enabled,
accessibility/speech-dispatcher also wants to pull in pulseaudio. Build it from ports without pulseaudio and you don't have to install pulseaudio for firefox.
Sadly more and more ports (unnecessarily) list pulseaudio as a dependency or are being bulit with it turned on. Because of this I've built more and more specific ports via poudriere and blacklisted pulseaudio there & via pkg on my clients to avoid it, but I recently switched to building everything via poudriere to prevent some occasional dependency pitfalls when using 2 repositories, and also to get rid of some other things or at least avoid them as much as possible (like dbus, avahi, akonadi, samba...).
BTW: if you build firefox without dbus support, it is MUCH more responsive (yet still bloated beyond repair and needs to be killed after ~2 days)
If you can't/don't want to build from ports and something refuses to install with pulseaudio being blacklisted,
rm
the pulseaudio executable,
ln -s /bin/true
in its place. this will truly prevent it from being started, as that crap doesn't respect any settings to disable it and even spawns a demon that isn't enabled via rc.conf.