Our bug tracker is hosted on Launchpad. It is an essential part of how the Mixxx team keeps track of and gradually eliminates all of Mixxx's defects.
If you wish to start working on a Bug you should adopt it by assigning it to yourself. This way other developers are informed that you will take care of it. If you decide not to finish the Bug, you should add a link to your unfinished work on GitHub or attach a patch. And than un-assign yourselves.