Change Java-Plugin Classpath?

Hello,

Is it possible to change the (boot)classpath of the Java-Plugin when I 
install a Mozilla extension? Since I use some custom classes, I want to 
start Java with something similar to

java -Xbootclasspath/p:/my/own/class/path/to/my.jar

so that my.jar is found.

Is this possible? Will it work on all plattforms?

Thanks a lot,
Keno