I had the reverse, only the gtk3 version was installed and firefox was complaining when started from an xterm. Using your description in reverse, 'apt-get install libcanberra-gtk0' made that error go away.
I had the reverse, only the gtk3 version was installed and firefox was complaining when started from an xterm.
Using your description in reverse, 'apt-get install libcanberra-gtk0' made that error go away.