Please edit your entry and change the title. As written, by itself, the title of this thread is meaningless. These are read by people in lists of threads, and this particular title is extraordinarily unhelpful.
You have two questions. A) Why does Firefox produce this message, and B) is there something not installed?
The best way to find out the answer to question "A" is to scan the source code. You sir, have the tools to do this.
Yourself. Because you have installed the ports tree. The following example commands will indicate where in the Firefox source code "drawable" is used. If it isn't found, then a dependency issued your warning message, and you will need to expand your search.
Code:
$ cd /usr/ports/www/mozilla-firefox
$ env NO_DEPENDS=Yes make patch
$ find /usr/ports/pobj/mozilla-firefox -type f -exec grep drawable {} /dev/null \;
Disclaimer: This was from memory; do not blindly type these commands into your computer. Understand what they do and modify accordingly.
If you need to know more, please see the appropriate man pages: cd(1), env(1), make(1), ports(7), bsd.port.mk(5), find(1), grep(1).
The answer to question "B" should be self-evident to the ordinarily prudent user of packages and ports. Ask yourself, "What is a dependency, and where does pkg_add get dependency information?"