After installing Panther and Macromedia Studio MX 2004 and using them for about 2 weeks, Macromedia Extension manager suddenly stopped working. Every single time I would try to start it, it would unexpectedly quit. It turns out the solution was to go into

~user/Library/Application Support//macromedia/dreamweaver MX 2004/configuration/extensions/

and disable all extensions by moving them to the "Disabled folder". I had to also restart the machine before EM got going again. Then you can test each extension by moving it back to see if Extension Manager still works.

Many thanks to DC_Kyote on the Macromedia Dreamweaver Support forums for this one.