GNU IceCat "Undo Close Tab" option not working.

I am using GNU IceCat 31.2.0 (which is installed by sudo apt-get installed icecat) on Trisquel 7.0 LTS.
Recently I found that "Undo Close Tab" option is not working. I also tried after restarting browser & OS.

Also checked "about:config" which is:

browser.sessionstore.max_tabs_undo;10
browser.sessionstore.max_windows_undo;3
dom.undo_manager.enabled;false


Then Why this option not working?