A wonderful add-on to do for your tabs what GeekTool does for your desktop. It delivers an easy to use interface to embelish your new tabs and at the same time make them more useful.
At the moment of writing the extension has been stable and worked correctly on Firefox 54.
Absolutely recommended :)

EDIT: One proviso though.
If you happen to have your Firefox Preferences set to "Never Remember History", Tabliss WILL forget all your settings between sessions. If that is your behaviour then, for the moment at least, Tabliss will be not much more than a beautiful picture, the time and a basic greeting every time you open a new tab.