Option renumber-windows was implemented in tmux 1.7, tmux in Debian 7 is 1.6.
set-option -g pane-base-index 1
# Renumber windows sequentially after closing any of them
#set-option -g renumber-windows on
-#Commented out - it doesn't work with my tmux.
-#Perhaps tmux in Debian 7 is too old.
+# Option renumber-windows was implemented in tmux 1.7, tmux in Debian 7 is 1.6.
# Attempt to set the client terminal title
set-option -g set-titles on