Those are the directory which the OS will search for binaries to execute when you type just the prog name.
Forefox and mozilla from Slackware-current packages is installed in /usr/lib/firefox-version and /usr/lib/mozilla-version respectively.
But it has a symbolic link from /usr/lib/firefox-version/firefox /usr/bin/firefox same goes for mozilla.
I hope you get what I meant
If not just ln -s /where/you/install/firefox/firefox /usr/bin/firefox