I have installed pulseaudio in order for steam to work, but do not have USE=pulseaudio on any packages. If I "lsof | grep libpulse" I can see that firefox is linked to libpulsecommon-9.0.so. I'm not sure if there is anything that can be done (it might just dynamically load it if it finds it), but I just thought I'd report and see. Reproducible: Always
pulse is dlopened if found now days. The ebuild and eclass will be updated to reflect this change as soon as 51.0 final is released.