See https://lldb.llvm.org/index.html.
Mozilla-specific lldb settings
There's an .lldbinit file in the Mozilla source tree, which applies recommended settings and includes a few type summaries and Mozilla-specific debugging commands via the lldbutils module (see python/lldbutils/README.txt). For information about available features see the links above and Using LLDB to debug Gecko blog post.
The in-tree .lldbinit should be loaded automatically in most cases when runnning lldb from the command line (e.g. using mach), but not when using XCode. See Debugging on Mac OS X for information on setting up XCode.
LLDB warning: Xcode 5 only comes with lldb (gdb is gone). The introduction and use of UNIFIED_SOURCES in the source starting around November 2013 has broken the default LLDB configuration so that it will not manage to resolve breakpoints in files that are build using UNIFIED_SOURCES (the breakpoints will be listed as "pending", and lldb will not stop at them). To fix this add the following to your $HOME/.lldbinit file:
# Mozilla's use of UNIFIED_SOURCES to include multiple source files into a # single compiled file breaks lldb breakpoint setting. This works around that. # See https://lldb.llvm.org/troubleshooting.html for more. settings set target.inline-breakpoint-strategy always
Restart Xcode/lldb and restart your debugging session. If that still doesn't fix things then try closing Xcode/lldb, doing a clobber build, reopening Xcode/lldb, and restarting your debugging session.
Starting a debugging session
Attaching to an existing process
You can attach to Firefox with following command:
(lldb) process attach --name firefox
Some versions of lldb causes crashes after attaching to Firefox.
Running a new process
To start Firefox under the debugger, run lldb followed by "--", followed by the command line you'd like to run, like this:
$ lldb -- obj-ff-dbg/dist/Nightly.app/Contents/MacOS/firefox-bin -no-remote -profile /path/to/profile
Then set breakpoints you need and start the process:
(lldb) breakpoint set --name nsInProcessTabChildGlobal::InitTabChildGlobal Breakpoint created: 1: name = 'nsInProcessTabChildGlobal::InitTabChildGlobal', locations = 0 (pending) WARNING: Unable to resolve breakpoint to any actual locations. (lldb) r Process 7602 launched: '/.../obj-ff-opt/dist/Nightly.app/Contents/MacOS/firefox-bin' (x86_64) 1 location added to breakpoint 1