Mature, well-established codebase
The first lines of source code were added to Mozilla Firefox in 2002. This is a relatively long time for an open source project to stay active, and can be a very good sign.
A long source control history like this one shows that the project has enough merit to hold contributors's interest for a long time. It might indicate a mature and relatively bug-free code base, and can be a sign of an organized, dedicated development team.
Note: The source code for Mozilla Firefox might actually be older than the source control history can reveal. Many projects begin by duplicating a large amount of source code from an existing, older project. You can usually tell whether this is the case by looking for a rapid rise in the amount of code early in the project's history.
Very large, active development team
Over the past twelve months, 727 developers contributed new code to Mozilla Firefox.
This is one of the largest open-source teams in the world, and is in the top 2% of all project teams on Ohloh.
For this measurement, Ohloh considered only recent changes to the code. Over the entire history of the project, 1786 developers have contributed.