Did you download mozilla source, a binary?
You only use the upgradepkg command if you are upgrading a Slackware packages with another Slackware package.
If its just a binary tarball, which is what I think mozilla usually distributes, just remove the mozilla package and unpack the mozilla tarball.
If you want a Slackware package of the new version of mozilla, get it from
In my experience though, it doesnt really matter if you do an upgradepkg or if you uninstall the old one and install the new version.