Do you have the Japanese font package installed on your computer? If not, then this would probably help:
- Go to Control Panel
- Go to Regional and Language Options
- Click on "Languages"
- Check the box that says "Install East Asian Fonts"
- Click "Apply"
- Click on "Advanced"
- In the dropdown, select Japanese
- Click "OK"
The final step is to change the time and currency into Japanese through the "Regional and Language" setting in the Control Panel. This won't work on Windows 98.
I'm not sure how to do it for just Firefox though, sorry