Lost around a thousand tabs and I don't know which.
When searching for a keyword to close tabs, as soon as I close a tab the list gets reloaded with other contet. So if I click on several close symbols in a row, tabzen will replace the list I am working with and put a category with all my tabs directly under the mousepointer, closing this category closes all tabs. Closing hundreds of tabs stalled Firefox and it restarted. The tabs already closed was thus no longer to be found in the "recently closed tabs" list.

Putting a button that means "throw all your stuff away" right under the mouse pointer while I am clicking it isn't a nice feature. It's nothing but deliberate and vicious.