The add-on, Bell of Mindfulness, delivers a bell that reminds us to breathe and be mindful of our body and mind.
Here is a little more about the add-on from the developers:
In the busy time of our daily life, we let our mind float away to many distractions. We forget about our body, our breath, and our mind; we forget about the unity of our mind and body. In order to remind us of this unification, the bell of mindfulness is meant for us to bring our mind back to our body. When we hear the sound of the bell, please stop what we are doing and take three breaths to bring our mind back to our body. Let this unification happen and be happy in the present moment.
Want to pick it up? You can get the Bell of Mindfulness add-on for Firefox from the Firefox Add-ons website.