Codetree has robust support for milestones and it's easy to get started using them.
- Managing Milestones - Quickly access Milestones from the left hand toolbar throughout Codetree
- From within Milestones, you can add a new Milestone at any time
- If you need to edit a milestone (name, start and end dates, etc.)
UPDATE Feb-2019: We currently have a bug attempting to edit start and end dates in some versions of Firefox. (see here)