netscape 4.0 anything won't be compatible with Threads.
Do a search for "netscape 4.7" at Infopop community and you'll see. It's support for CSS is spotty - and Threads uses CSS exclusively.
On my mac - things looked good in Mozilla.
In Safari - the layers worked - but the box to display the stuff was limited to about half the size of the screen - thus it ended up covering the legend.
On the mac in OmniWeb - when I clicked the layers appeared at the very top of the screen and covered up the logo.
On the mac in IE - it worked fine - but the area where the info appeared was wider than the screen and caused the whole screen to scroll left/right.
Probably it would help if you made the page validate at w3.org
Right now there are alot of issues - and cleaning those up might make it work better:
See Here