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
http://www3.linuxpackages.net/packag...obert/mozilla/
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.