Jan and I just added MySQL support to Mozilla's database component. It isn't part of the default Mozilla build, but you can compile it yourself, or use one of the XPI's, although they haven't been updated yet and seem to be broken links anyway.
I plan on writing some documentation on the component, but basically, it allows you to connect directly to a database. The component also supports a datasource for displaying results in a template or tree. Currently, the sql component supports PostgreSQL, MySQL and SQLite.