This has been removed in v1.2.3
WP-SCSS uses scssphp which, as far as I can tell, does not yet support sourcemaps. There is some discussion around it being a WIP, but we can’t add this feature until it’s been added to that library.
Is there a specific issue you are having? Maybe I can help.
I suspect not, wp-scss uses a php compiler built on the libsass library. I assume firesass requires ruby. I’ve never used firesass before so there maybe some way you could get this to work. Let us know if you find something!
Thanks for pointing that out. Just released a quick fix in 1.1.2. Great suggestion!