I beg to differ.
In that bug report, the firefox devs say to just turn off the system bell in gnome (similarly in KDE). I don't run gnome. I don't run KDE. I tried installing gnome just to turn off sound. Nope. I tried installing KDE just to turn off sound. Nope.
Here's what I ended up doing:
emerge --unmerge sound-theme-freedesktop
USE=-sound emerge gnome-media
The second was to prevent gnome-media from pulling in the freedesktop sound again. Presumably now, if I did use gnome, I'd have no sounds at all.
Hey Firefox, how about a config:option. I shouldn't have to neuter my system. Stop interfering with system-level alerts.