Rated 5 out of 5 stars

Awesome! I hope it will evolve continously:)
H:MM:ss format is good tip;)

This review is for a previous version of the add-on (0.4.13). 

Thank you for the review. On H:MM:SS, I decided to only show minutes and not seconds in order to keep the add-on lightweight and not impact browser performance. Updating by the minute is much lighter than updating every second. Also, I think it makes sense to measure the time spent by minutes and not seconds. (Note: it still keeps accurate time, to the second, but only updates the ticker by the minute.)