This is a free extension which is a Firefox version of the Chrome extension of the same name (by a Dutch firm called Q42). See ScrumForTrello.com for more history.
This extension is very useful for Scrum projects which are using Trello it provides estimates and allows you to record time-spent.
My team created a different Trello add-on called "Burndown for Trello" which creates burndown charts for Trello boards. Since many of our users prefer to enter estimates right in Trello, instead of through our site, it's in our best interest to keep Scrum for Trello well-maintained and to make it accessible to more users (instead of just Chrome users).
The two extensions work beautifully together and will hopefully help your Scrum / Agile team be more productive!