Drivers provide project management for mozilla.org milestone releases. The drivers provide guidance to developers as to which bug fixes are important for a given mozilla.org release. Drivers also make a range of tree management decisions. The drivers are particularly active after the trunk is frozen for a milestone release, and in managing the milestone branch until a milestone is released. During this time the the Drivers watch the checkins very closely, and generally require that every patch be reviewed by the drivers before it is checked in. This review is not focused on the technical merits of the code itself. We use review and super-review for that, and the drivers expect that a fix proposed for a milestone branch has already been reviewed and approved by relevant reviewers. Instead, the drivers review focuses on the importance of that particular fix to the milestone release, and the risk to stability and performance presented by that particular patch. So a driver may say "This is a great patch, please make sure it's checked into the trunk right away. But our milestone release is tomorrow, we're currently waiting for two identified patches before we release the milestone, and so don't check this patch into the current milestone branch."
So being a driver requires having the time, willingness and ability to focus on checkins to the Mozilla tree. Most but not all drivers are programmers themselves. Drivers need to be intimately familiar with the tree: when patches are checked in, which patches caused the tree to go red, which patches caused regressions, interactions between patches, dependencies among patches, and progress on fixing critical patches.
This level of involvement isn't required all the time, but is required from the time the trunk freezes for a milestone until that milestone is released.. Not every driver participates equally in each milestone, or each day of each milestone. For example, a Mozilla milestone release might come at a particularly inconvenient time for a driver. Perhaps his or her employer has a release scheduled for the same time. Or perhaps the driver has exams to take. In that case, we ask the driver to let the others know that s/he will not be available for that release.
The current list of drivers can be found in the project management section of the Mozilla Development Roadmap.