download english version of Firefox from here

• www.mozilla.org/en-US/firefox/all.html

Thanks for trying but if I do this won't everyone else using the computer get the english version. Isn't there some way of switching between languages?

You can install additional language packs.

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

Firefox Win32 language packs:

• http://releases.mozilla.org/pub/mozilla.org/firefox/releases/latest/win32/xpi/
• http://releases.mozilla.org/pub/mozilla.org/firefox/releases/latest-3.6/win32/xpi/

• Locale Switcher: https://addons.mozilla.org/firefox/addon/locale-switcher/

Thanks for trying but thisstill makes no sense to me, I can see how to install an english version but I think everyone will then get the english version. How do i switch between versions. Is this what the locale switcher is - it means nothing to me???

Which language pack you get is set via the value of the general.useragent.locale pref providing that language is installed in the profile folder that is used.
So for a user to get another language you need to install the language pack for that user and select that language either via the extension or by setting the general.useragent.locale pref to cy via user.js or prefs.js.

• user_pref("general.useragent.locale", "cy");

You can also do a custom installation of the Welsh version in a separate program folder and start that version if you need the Welsh version.

• https://support.mozilla.com/kb/Custom+installation+of+Firefox+on+Windows
• http://kb.mozillazine.org/Shortcut_to_a_specific_profile
• http://kb.mozillazine.org/Using_multiple_profiles_-_Firefox