Q: When do builds come out?
A: There are nightly builds and milestone builds.
Nightly builds are snapshots of the source code, automatically built using a script. They may contain bugs, which may or may not be severe. If you are uncomfortable with that, download milestone builds. The latest release is Firefox 0.9.3
Q: What is the difference between Branch and Trunk?
A: The source code used to build firefox is held on a database with a tree like structure. New features, APIs, untested code, is added to the trunk first. Branches are copies of the trunk taken near milestone releases. Usually, new features are added to the trunk first. Branches are feature-stable and most work concentrates on bugfixes in order to ship a milestone build.
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So, as a rule of thumb:
Trunk - bleeding edge features, with all the bugs you could ask for - daily builds
Branch - more features than a release, fewer bugs than trunk - daily builds
Release - you get what you get - one-off build