DTrace is Sun Microsystem's dynamic tracing framework that allows developers to instrument a program with probes that have little to no effect on performance when not in use and very little when active. Probe data can be collected with scripts written in D (no, not that one). Mozilla DTrace support has been added by the Sun DTrace team and can be used on Solaris 10 and Mac OS X 10.5.
The DTrace probes currently built into the codebase may be enabled by default in the future, but for now you'll need to create a build with --enable-dtrace (on Mac OS X you also have to use at least the 10.5 SDK for
--with-macos-sdk, unlike the common configuration that uses the 10.4 SDK).
Trunk nightly builds with shark support (*-mac-shark.dmg here) are built with --enable-dtrace.