You cannot navigate into pages in the DjVu document in FireFox on Mac OS X.

DjVu Plug-in Viewer 6.x for Mac OS X only supports Safari 3 and higher, and it doesn't support FireFox and other Mozilla compatible browsers. So, some functions of DjVu Plug-in Viewer won't work properly in FireFox. Especially the navigation toolbar buttons don't work properly.
There is a work-around to navigate into pages in the DjVu document on FireFox. In order to navigate within Firefox, the user must instead perform a right-click in a document and thereby pull up a "Navigate" drop-down list, which then allows advancement to other pages, zooming, etc.