Motitab от Peter

Replaces the new tab page with an ever incrementing counter of your age. Once installed, enter your date of birth via the add-on icon in your browser bar, and then every time you open a new tab, a live calculation of your age will appear.

This is similar to the "Motivation" add-on (https://addons.mozilla.org/en-US/firefox/addon/motivation-new-tab/) but doesn't strain your CPU unnecessarily, and is open source: https://github.com/VaguelySerious/motitab