Asking, because it took a minute to figure this out and nothing came up on Google. Answer coming up.
I downloaded a Firefox (nightly) build from mozilla.org for testing. Running it presented an error:
$ firefox XPCOMGlueLoad error for file /home/parallels/firefox-32/libmozgtk.so: libgtk-3.so.0: cannot open shared object file: No such file or directory Couldn't load XPCOM.
The file does exist on my Ubuntu 14.04 LTS:
$ locate libgtk-3.so.0 /usr/lib/x86_64-linux-gnu/libgtk-3.so.0 /usr/lib/x86_64-linux-gnu/libgtk-3.so.0.1000.8