(you'll need to have root access or sudo in order to use apt-get install)
Note the xfonts ones are smaller in size (but didn't help in my case)
You should already be able to see the japanese fonts in your browser. If this doesn't work, try the following in any order:
restart your browser (close every window before launching it again)
go to the extensions page for Firefox and follow the link to the "Languages" extension's homepage, then follow the link to the language-specific extensions and install the Japanese one (strange characters ending with "Ja")
restart your X server (in order for him to take into account the new fonts - I really don't think this is needed, but just in case...)
Installing the "Languages" extension
Start Firefox and go to the extensions handler (Tools -> Options -> Extensions) and click on the link to get to the extensions' page.
Select "Install" on the "Languages" extension section. Install the extension by clicking "Cancel" if you're not administrator so that the extension is installed only for your user. Clicking "OK" when you're not administrator will probably not succeed in installing the extension.
Then you'll have to restart your browser in order for your changes to take effect.