Ok. There should be a space between /usr/bin/firefox and /usr/lib/firefox. So your executable should be in /usr/bin/firefox. This is the default location for a slackware installation.
As far as a new link, I actually can't help there as I don't use KDE. I would check in the KDE admin browser for a way to edit the menus. Otherwise I'm sure someone will come along here with the knowledge.