This is a quick update on the status of Firefox's support for source maps.
The library for parsing and generating source maps has landed in Nightly! If you
want to write an addon which either uses or creates source maps, import the
source map module with
documentation for the source map module is on GitHub. The history is
inside bug 669999.
What I am working on now is writing the plumbing between our script loaders, SpiderMonkey, and the debugger API to shepherd the source map URL for a given script to our developer tools. I have working patches for most of this, it is "just" a matter of writing test cases, getting proper review, updating patches based on review feedback, and constantly rebasing dependent patches on top of said updates.
You can follow along with the plumbing in these bugs:
Once that plumbing is complete, adding source map support to the debugger's UI should be pretty straight forward since all the required data will already be available. You can follow along with that final piece on bug 771597: Integrate source maps with the debugger.