I'm so sorry about your loss. So you pressed the "Close this window" button ( when you hover over with the mouse, it also says "Close this window" - and the text in the top of the extension also changes to "Close this window" )

But you assumed it would close the sidebar, is that right? And you had one window open with 415 tabs?

Argh.

Were you able to recover the tabs by going to the firefox history, and reopen them?

To be honest, adding an additional step when closing a window would also reduce the simplicity of the extension. One of the nice features about it is that you can quickly close many windows while cleaning up your workspace.