I thought I'd share this here, as it's something to trip up the unwary. Posted here because there are many Debian/ Knoppix/ Ubuntu users in the Perl community.
I've had need to use a web based chat client for accessing a vendor's support facility, which needs the Java plugin. Fine, I go to the relevant page, which appears complete with jigsaw icons and the "Install Missing Plugins" button.
Sadly, it's one of those plugins that doesn't install at the click of a button, so I'll list the details of what needs to be done to install it.
First, there's a download page. Choose the second option: "Linux (self-extracting file)". Keep track of where you have saved this, e.g. non-root user's home directory or desktop.
Now, there are several ways of proceeding, but I present one method which gives an upgrade path. For this, you need to have the Debian packages fakeroot and java-package, together with any dependencies installed.
Issue the following commands
fakeroot make-jpkg [jre file you downloaded]
This will prompt you with a "proceed" question, and also present you with the full text of the license, which you need to scroll through and agree to. This results in a debian package, which you then install with:
sudo dpkg -i [package]
Run the command java -version to check that the installation has worked. Now you need to symlink in the plugin, so that Firefox can see it. Plugins are kept in a directory off ~/.mozilla called plugins, which might not exist; create it if it doesn't exist, then place a symlink to the plugin in this directory
Now, you need to restart Firefox, and you should pick up the plugin.