Difference in rendering web control when using Firefox?

[Herman]

I have build a web application as a web user control. This app is
hosted / loaded in a WCMS application. The application itself uses a
start.ascx to dynamically load the necessary controls. Schematically:
Hosting WCMS application
Application Start control
query form
list form
detail form
etc..
This runs smoothly on my development environment when I run the
control in an otherwise empty default.aspx, for IE as well as Firefox.
However, when I load it in the WCMS environment it runs perfectly fine
in IE, but Firefox throws a server error (Failed to load viewstate). I
could do with a few pointers, because I'm running out of options
here..
The url in question is
http://www.cgn.wur.nl/UK/CGN+Plant+Genetic+Resources/Search+and+order+germplasm/On+line+search/,
the error shows up after doing a search in a search form.

TIA,
Herman