non-gecko-based browser ??? Are there any for FC1 ????

[Björn Persson]

That online store was probably written for Explorer only. A wild guess 
is that there was some Javascript code that used something Explorer 
specific.

When you write to webmasters, don't just ask them to make their pages 
Mozilla compatible. Ask them to follow the standards:

http://www.w3.org/TR/html4/
http://www.w3.org/TR/xhtml1/
http://www.w3.org/TR/CSS2/
http://www.w3.org/DOM/
http://www.ecma-international.org/publications/standards/Ecma-262.htm
(Never heard of ECMAscript? It's more known as Javascript.)

W3C's validators are very handy:
http://validator.w3.org/
http://jigsaw.w3.org/css-validator/

Björn Persson