(In reply to (not currently active) Ted Mielczarek from comment #1)
I poked around a bit and I'm not 100% sure you can use this the way you want to. It seems to be primarily focused on allowing you to add some additional context to WER reports.
I manage to coerce it into writing minidumps via the callbacks that are in the runtime exception module. I haven't tested every possible exception yet but it looks quite promising and I hope it can capture most of the crashes that we're missing. A very nice side-effect of this is that it all works out of the crashed process and while you're in WER the process is suspended, so I'm almost hopeful to be able to disable Breakpad's exception handler once I get it workig.
Additionally, I ran into this MSDN article about enabling User-Mode dumps in WER, which discusses how you can configure registry keys to ask WER to write minidumps to a local path on disk, including configuring that on a per-application basis. If writing dumps from the runtime exception module doesn't prove feasible, setting these keys in the installer might be a way to still get minidumps from crashes that Firefox can't handle itself and upload them to crash-stats.
I've considered that, but only as a last resort. The problem is that there's nothing to inform you that the minidump was written so you'd have to poll the target directory after a process crashes hoping to find something.