Flash and therefore YouTube wasn't working in Firefox 3 RC on Slackware 12.1. The solution? Install flash player from the SlackBuilds repository. Then, cd into the flash installer and copy the .so file to /usr/lib/firefox-3.0rc2/plugins/. Restart Firefox.
And to upgrade a program using an updated slackage, fred on Freenode suggests: # upgradepkg --reinstall --install-new slackage.tgz