You will need to leave one tab open or close firefox. You could drag one to a new window, and close the original window. Alternatively if you right click a tab you get the option to close other tabs.

If you turn off the option to open new windows as tabs that may help.

• use Firefox button -> Options |Tabs |
  [] Open new windows in a new tab instead
• see Tab preferences and settings

In addition to the ordinary bookmarks note the new feature (especially useful for visual and graphic content ) what are app tabs?

There is plugin that I recommend you work with him as the "Close All Tabs (Reloaded)" Designed exactly for what you are asking here