Flash 12.0 ? Current flash version for Linux is 184.108.40.2060, and Adobe says 11.2 is the end for Linux.
The latest Seamonkey patch for Slackware 14.0 is 2.17.1, by the way. If you are using the one released with Slackware 14, you should probably update it. It works for me with Flash 11.2, with the file installed as you said above.