When a remote policy file is being used in java.security then firefox will
hang as soon as any applet is being accessed. This worked for 6u3 and earlier.
The bug was introduced in 6u4 and is still there in the latest 6u10beta of May1st.

This is reproducible always on Windows Steps to reproduce:

Install 6 u4|5|6|10beta
Make sure that the plugin is configured for both
IE and Firefox.

Then modify jre\lib\security\java.security like this:

policy.url.1=file:${java.home}/lib/security/java.policy
#policy.url.2=file:${user.home}/.java.policy <<<< comment out
policy.url.2=http://aubing.germany/ourpolicy.policy <<<< insert

The remote policy file normally contains some grant statements.
For testing, just an empty remote policy file will do.
The link above to aubing.germany points to the empty file
ourpolicy.policy.

Start IE and access load any demo applet such as
http://java.sun.com/applets/jdk/1.4/demo/applets/Clock/example1.html
This will work as expected.

Then do the same with firefox. Watch the hang. A thread dump is
attached