FULL PRODUCT VERSION :
java version "1.6.0_18"
Java(TM) SE Runtime Environment (build 1.6.0_18-b07)
Java HotSpot(TM) 64-Bit Server VM (build 16.0-b13, mixed mode)

ADDITIONAL OS VERSION INFORMATION :
Windows XP Professional x64 Edition [Version 5.2.3790]
Windows Vista
Ubuntu 8.04

EXTRA RELEVANT SYSTEM CONFIGURATION :
Firefox 3.6.x is the system default browser

A DESCRIPTION OF THE PROBLEM :
If Firefox 3.6.x is the default system browser and the following option is checked:

Edit | Preferences | Advanced | Network | Settings... | "Use system proxy settings"

then, any attempt to start an application through Java Web Start by clicking on a JNLP file in a web page fails.

STEPS TO FOLLOW TO REPRODUCE THE PROBLEM :
1. set Firefox 3.6.x (it seems to occur with 3.x, too) as the system default browser
2. check the following option in Firefox:
Edit | Preferences | Advanced | Network | Settings... | "Use system proxy settings"
3. try to start an application through Java Web Start, for instance:
http://java.sun.com/docs/books/tutorialJWS/uiswing/learn/ex6/CelsiusConverter.jnlp

EXPECTED VERSUS ACTUAL BEHAVIOR :
EXPECTED -
The application starts
ACTUAL -
The application does not start, an error is given by the Java Web Start client.

ERROR MESSAGES/STACK TRACES THAT OCCUR :
java.lang.IllegalStateException: DynamicProxyManager: Invalid Proxy Type
at com.sun.deploy.net.proxy.DynamicProxyManager.reset(Unknown Source)
at com.sun.deploy.net.proxy.DeployProxySelector.reset(Unknown Source)
at com.sun.javaws.Main.initializeExecutionEnvironment(Unknown Source)
at com.sun.javaws.Main.continueInSecureThread(Unknown Source)
at com.sun.javaws.Main$1.run(Unknown Source)
at java.lang.Thread.run(Unknown Source)


REPRODUCIBILITY :
This bug can be reproduced always.

CUSTOMER SUBMITTED WORKAROUND :
In Firefox, either select "No Proxy" or set up the appropriate proxy settings.

Other details for Ubuntu can be found here:
https://bugs.launchpad.net/ubuntu/+bug/375194