There are some terms that are used by the Mozilla community. One sees them on IRC or in bugs. It can be helpful to know exactly what these refer to. Note that this is an attempt at describing the current usage of these terms. This usage may change.
A trunk build is a build made off the trunk, i.e., the latest, newest, but also least stable and most buggy state of the source. Trunk builds are provided so that one can execute tests without going to the trouble of building the source.
For the trunk and the latest two branches, there are machines at Mozilla and maybe elsewhere compiling builds as fast as they can make them for at least all three of Firefox, Thunderbird and SeaMonkey on all three of Linux-i686, Mac-UniversalBinary and Win32. These are the "tinderbox builds", also known as "hourly builds" though it usually takes more than one hour to make one; they are followed by automatic tests and their main purpose is to check that nothing is horridly wrong with the latest change to the source: what they are doing is to constantly check that the current source can be built into an executable and that that executable passes a certain more-or-less fixed set of tests.
The same machines produce about every 24 hours a "nightly" build which can be downloaded by anyone if you know where to get it: these are the nightly builds, and testers all over the world download them and test them, reporting as they go along on any bugs that hit them. Anyone can be a tester, you don't need to get approval before you can start testing. When people talk of "nightlies", they often mean "trunk nightlies", though there exist also "branch nightlies" which are somewhat less buggy but also not as "newest & greatest". For instance, one may use a Thunderbird "branch nightly" as one's main e-mail client.
Nightly builds can be downloaded from http://ftp.mozilla.org/pub/mozilla.org/firefox/nightly/.