Interesting question. I tried compiling Firefox with 1) the above line in the mozconfig file, 2) replacing the gtk3 with gtk2, and 3) removing the line altogether. Firefox compiled without the above parameter looked exactly like when you compile with the --enable-default-toolkit=cairo-gtk3 option. And of course, the 2'nd compile used the gtk2 toolkit.
So I'm assuming it's safe to leave the line out now.
Hope this helps.