Here’s how I’m currently debugging Electrolysis stuff on OS X using gdb. It involves multiple terminal windows. I live with that.

# In Terminal Window 1, I execute my Firefox build with MOZ_DEBUG_CHILD_PROCESS=1.
# That environment variable makes it so that the parent process spits out the child
# process ID as soon as it forks out. I also use my e10s profile so as to not muck up
# my default profile.

MOZ_DEBUG_CHILD_PROCESS=1 ./mach run -P e10s

# So, now my Firefox is spawned up and ready to go. I have
# browser.tabs.remote.autostart set to "true" in my about:config, which means I'm
# using out-of-process tabs by default. That means that right away, I see the
# child process ID dumped into the console. Maybe you get the same thing if
# browser.tabs.remote.autostart is false. I haven't checked.

CHILDCHILDCHILDCHILD
  debug me @ 45326

# ^-- so, this is what comes out in Terminal Window 1.

So, the next step is to open another terminal window. This one will connect to the parent process.

# Maybe there are smarter ways to find the firefox process ID, but this is what I
# use in my new Terminal Window 2.
ps aux | grep firefox

# And this is what I get back:

mikeconley     45391  17.2  5.3  3985032 883932   ??  S     2:39pm   1:58.71 /Applications/FirefoxAurora.app/Contents/MacOS/firefox
mikeconley     45322   0.0  0.4  3135172  69748 s000  S+    2:36pm   0:06.48 /Users/mikeconley/Projects/mozilla-central/obj-x86_64-apple-darwin12.5.0/dist/Nightly.app/Contents/MacOS/firefox -no-remote -foreground -P e10s
mikeconley     45430   0.0  0.0  2432768    612 s002  R+    2:44pm   0:00.00 grep firefox
mikeconley     44878   0.0  0.0        0      0 s000  Z    11:46am   0:00.00 (firefox)

# That second one is what I want to attach to. I can tell, because the executable
# path lies within my local build's objdir. The first row is my main Firefox I just
# use for work browsing. I definitely don't want to attach to that. The third line
# is just me looking for the process with grep. Not sure what that last one is.

# I use sudo to attach to the parent because otherwise, OS X complains about permissions
# for process attachment. I attach to the parent like this:

sudo gdb firefox 45322

# And now I have a gdb for the parent process. Easy peasy.

And finally, to debug the child, I open yet another terminal window.

# That process ID that I got from Terminal Window 1 comes into play now.

sudo gdb firefox 45326

# Boom - attached to child process now.

Setting breakpoints for things like TabChild::foo or TabParent::bar can be done like this:

# In Terminal Window 3, attached to the child:

b mozilla::dom::TabChild::foo

# In Terminal Window 2, attached to the parent:

b mozilla::dom::TabParent::bar

And now we’re cookin’.