If a directory for the locale the browser is running in doesn't exist, Firefox will automatically try the 'default' locale, as defined by the distribution.searchplugins.defaultLocale preference.
To set a given search engine as the default, you need to use <code>LocalizablePreferences</code> to override the <code>browser.search.defaultenginename</code> preference. You should also override the <code>browser.search.order.1</code> preference to put the default engine in the right sort order. For example: