It won't find updates for most extensions because they "aren't compatible" - that's a bit of an anomaly that I doubt there's any way to fix (Firefox gets a load of XML and doesn't see anything for a version beginning 3.1, decides there's no updates, or tells the server it's 3.1b1 and gets an empty response).
Find Updates will only, as the name suggests, find updates for extensions you already had installed...