This works out of the box in Chrome, so seems an odd omission in Firefox so I'm assuming I've done something wrong.
There is a seperate article about source map support in Firefox that elaborates on the minimum required Firefox version for sourcemaps to work (Firefox 29).
Also, keep in mind that not all SASS-generated files come with sourcemaps - this is a feature that must be manually enabled with the
--sourcemap argument if you're using it from the command line, by rendering it with
render_with_sourcemap from within a ruby program, or by other means if you're using a task manager such as gulp or grunt.
You can check if your stylesheet has an associated sourcemap by looking at the last non-empty line of your generated css file - it should look somewhat like this:
/*# sourceMappingURL=style.css.map */