Don't hide the tab bar even if there is only one tab, on Firefox 23 and later (#541)

Because the feature is removed on Firefox 23.
https://bugzilla.mozilla.org/show_bug.cgi?id=855370
However, the addon "Hide Tab Bar With One Tab" still provides it.
https://addons.mozilla.org/firefox/addon/hide-tab-bar-with-one-tab/
So, I decided to activate the behavior only on old Firefox or the environment which has the addon.