Google search; not country-specific, but language-specific. Russian language, to be precise. Searching with this plugin only returns Russian-language websites for results.
The previous 2 versions of the plugin not working in Firefox 3.5, the problem was in the presence of a bug, which is not in Firefox 3.6. Editors AMO confirmed the existence of the bug. To avoid such problems would recommend to use only the latest version of Firefox.