Hi Eddie,
I haven't checked this rigorously in a while, but I think the story goes like this:
The "advanced" features of the web export, such as the very-nice Pan & Zoom, and the Search functionality depend on a vector representation of the exported diagram. This vector representation is created via VML, a just-about-dead vector language that only runs in Internet Explorer, as far as I know. Ah propriety!
If the VML capability is not present, the display reverts to the 'backup bitmap', which is usually a .jpg, but can probably be specified under the web-export options. You'll still get hyper-linking hot zones, but I think a lot of the other stuff goes away.
Perhaps there is a VML plug-in for FireFox, but I don't know if even that would solve the problem.
