You can install extra language packs and set the general.useragent.locale pref to the language code (ja for Japanese) on the about:config page.

You can install such a language pack in the profile folder or possibly globally with a little more effort to have them for all users (profiles).
Best is to install the en-US version of Firefox in this case.

Firefox Win32 language packs:

• http://download.cdn.mozilla.net/pub/mozilla.org/firefox/releases/latest/win32/xpi/

See also "-UILocale":

• https://developer.mozilla.org/Command_Line_Options
• http://mike.kaply.com/2013/02/11/add-ons-in-the-distribution-directory

Thanks...but is there any way to install two separate instances? I'd like to be able to customize...she likes themes and I don't, for instance. I like seeing a really clean interface and she like toolbars...I'm hoping we don't both have to use the same browser.

Make sure that each of you has their own Firefox profile folder.

See:

• http://kb.mozillazine.org/Creating_a_new_Firefox_profile_on_Windows
• http://kb.mozillazine.org/Shortcut_to_a_specific_profile
• http://kb.mozillazine.org/Using_multiple_profiles_-_Firefox

You can add -no-remote to the command line to open another Firefox instance with its own profile and run multiple Firefox instances simultaneously.

• http://kb.mozillazine.org/Opening_a_new_instance_of_Firefox_with_another_profile

Thanks!...will give it a whirl!