I started getting this problem yesterday, though I remember experiencing the same issue in Firefox 3, I think. It starts when I close a tab: it seems to be closed but if I press ctrl+tab I can still see it. After it starts every tab has the same behavior. Old tabs won't close, new tabs won't appear on the tab bar - but they still exist and can be reached by ctrl+tabbing my way to them - and I get this huge memory leak, hitting close to 1GB ram with 6 tabs, most of them are these unwanted ghost tabs. The only way to "solve" it so far is stopping everything I'm doing and restarting Firefox. Any tips?