Figuring out the dependencies has been a major hassle due to Void's unique packaging and naming strategies, but here is my very over-inclusive list* of packages I installed before it would eventually build -
glib-devel gdk-pixbuf-devel pango-devel pango-xft binutils base-devel python-dbus-devel libXt-devel yasm dbus-glib-devel libdbusmenu-glib-devel autoconf213 gtk+-devel GConf GConf-devel pkgconf pkgconf-devel alsa-lib-devel openssl-devel sqlite-devel pulseaudio-devel gtk+3-devel libgtkdgl libgtkdgl-devel gegl-devel
Other than that, it builds on Void with the normal git commands from the Pale Moon Linux Build page.
I built it without the Pale Moon branding just to see what it looks like. The start page for the unbranded version is quite good looking.
*(I'll try to figure which of these dependencies aren't necessary and update this thread with a more minimal list later).