I installed the new Diigo Toolbar 3.1.5.4 and now the diigo toolbar points to http://secure.diigo.cn/sign-in.
I had the same problem with the last Version.
I found the error in the file: diigo.urls.js in the first lines:

(function(){var A=window.diigo;A.urls={matrix:{cn:{www:"http://www.diigo.cn",group:"http://group.diigo.cn",dms:"http://message.diigo.cn",toolbarServer:"http://www.diigo.cn",signIn:"http://secure.diigo.cn/sign-in",signOut:"http://www.diigo.cn/sign-out",resources:"http://resources.diigo.cn",cookieDomain:".diigo.cn"},

I changed all diigo.cn into diigo.com and everything works now.

I run Diigo Toolbar on Firefox 3.0b5 and OSX 10.5.3

I have the same problem with Firefox 3.0b5 and Diigo Toolbar 3.1.5.2 on Mac OSX 10.5.2