All of the searches listed in the toolbar search box are by default all US based. You are given the option to add new search engines to this list. However, you are not given the option of removing items from this list. Below, you will find removal instructions:

Windows: Go to C:\Program Files\Mozilla Firefox\searchplugins (I don't know if Mac will be different).

Each search engine has two files; .src and either .png or .gif. All you have to do is delete the two files (for example google.src and google.gif) and restart Firefox.