FireSass is a Firebug extension that makes Firebug display the Sass filenames and line numbers of Sass-generated CSS styles rather than those of the generated CSS.
First, install FireSass.
Second, enable Sass's :debug_info option.
If you're using Sass with a Ruby web framework, depending on the version,
you probably want to do:
config.sass.debug_info = true
for Rails 3.2, or for earlier versions of Rails
Sass::Plugin.options[:debug_info] = true
Add this to config/environment.rb in Rails,
or config.ru in other Ruby frameworks.
If you're using Compass, instead do:
sass_options = {:debug_info => true}
Add this to config/compass.rb, or wherever else your Compass configuration file is.
If you're using Sass from the command line,
just pass in the --debug-info flag.
Finally, delete all the existing CSS files
so that they'll be regenerated.
FireSass requires Sass 3.0 or later.
FireSass should work with all versions of Firefox after and including 3.0, and all FireBug versions 1.4 and 1.5. It might work with FireBug 1.6 (which is in development at time of writing), but that's not guaranteed.
FireSass currently requires the development version of Sass, available from GitHub.