Not sure how long ago you tried, but in one of the recent Firefox versions, they greatly improved the source map support - so it might just be that you should give it another try.
I've tried it with the latest versions (as of now), but it doesn't seem to match up with how good Chrome is at it. I've tried both normal Firefox and the developer version.
Why is this something Firefox seems to be struggling with? All the Blink based browsers seem to be much better in this regard.
