I fixed this by clicking on "Extensions" tab in the Add-ons manager and then back on Get Add-ons. It seams that Get Add-ons needs some time to load all the necessary data.

This fix is not working for me. It just spins and spins and I cannot see my Add-ons list. It is difficult to imagine why clicking 'Get Add-ons', which sets off a process requiring interaction with a remote database, should cure a problem with the Extensions list, which presumably sits locally.

The Yahoo Toolbar was apparently causing me the same problem with add-on manager just spinning and never completely loading. I clicked the "recent updates" and the add-ons showed up. I removed the Yahoo toolbar which is what I wanted to do anyhow and now add-on manager works fine.