OH!! this same thing happended to me! it was a while ago, so i may be missing a step. hopefully somone else will be able to confirm this post.
i needed to use the terminal to fix it. it may be helpful to use your file manager along with the terminal, just so you can visualize what directory you're in. you'll also need root access. the command su will let you get it. be ultra super carefull not to delete anything as root, or else.
I'm guessing you ended up at
http://java.com/en/download/linux_manual.jsp and don't know where to go.
It was pretty intimidating at the time, but it really isnt a problem. Just follow the instructions. one thing to keep in mind is that the instructions say jre1.5.0 a lot, when in reality, you probibly arent using that version. instead of typing the 1.5.0 part, use the numbers from the file you downloaded.
there are two main parts to the process. the first is to install the plugin, and the second is to link it to mozilla.
installing things in linux is nothing like installing things in windows. there isn't a registry in linux. in fact, with linux it's often just a case of getting the proper files into the right directory.
obviously, you need to download the thing. i was using fedora, so i went with the .rpm link. you end up with a file that ends in .bin. our goal for this first part is to get the files inside that .bin into their home -- /usr/java/jre1.5.0
next, you need to run it. you may have to make it executable, i cant remember if i had to. clicking it should work. it will make a file -- jre-1_5_0-linux-i586.rpm. running
this file will put the plugins where they belong.
ok, so on to the second part. we need to make a symbolic link in the firefox folder. a symbolic link is sort of like a shortcut in windows, except that programs can follow it too. first we need to get to your firefox directory. it may be in /usr/lib/mozilla. or it could be in /usr/lib/firefox. you may even have both, or more than one of either, with different version numbers. you will know if you are in the right one, because it has a subdirectory called /plugins. cd into that one.
now, it's time to make the link. type
CODE
ln -s /usr/java/jre1.5.0/plugin/i386/ns7/libjavaplugin_oji.so
ln tells your computer you want to make a link. -s tells it what kind of link to make (we want a soft link). the /usr/java... part says exactly what file we want to link to. remember to put in the right version number. if you look at this folder using your file manager, you should be able to see weither or not the link is broken. if it is, obviously something went wrong.
the last step is to select the option to enable java (in firefox) and then restart firefox. hopefully everything will have worked out. sorry my instructions are so vague, and i hope this helps