Just wanted to post this in case others have this problem. In Firefox I get this error:
ReferenceError: event is not defined
tb_show(jQuery(this).attr('title'), event.target.href, false);
Seems like it should be e instead of event since that's the parameter that was passed in. After changing to the line below it works.
tb_show(jQuery(this).attr('title'), e.target.href, false);
Thanks for the great plugin. It's very useful!