There are slightly different viewers for the Firefox extension, Chrome extensions and B2G (Boot to Gecko, a “standalone operating system for the open web”). While the web viewer is updated almost everyday by a large group of contributors, these extensions receive far more infrequency updates (about every two weeks) by a small core group.
Structure and Guiding of Work
Guiding the progress of the library is done through milestones on Github. These milestone set out a specific goals for a release, such as “Text selection and search” or a specific Firefox release. There have also been special requests, like Font testing, on the mailing list. Of course contributors are free to work on any issues they please, but merging those issue will be less of a priority or might be delayed to the next milestone.
As for the progress of my contribution, it has gotten caught up in a very active discussion on the how to handle user preferences. I am following this discussion closely and once this large change to the viewer has been merged, I will update my contribution to autohide the toolbar to match the changes.