Of interest: Collaborative and Social Tagging Networks (2008-01) included France and China.
When I last checked (probably around 2009-03) there was nothing more recent that reminded me of Diigo's feature set. Could be good to have Diigo covered in a future article.
It's possible for extensions to Firefox to be in conflict with each other.
Uninstalled version 3.1.6.16 of the add-on, quit from Firefox, started Firefox, installed version 4.1.0.3 from http://www.diigo.com/tools/toolbar, restarted Firefox