How can I set sourcemaps for sass files on Firefox - Firebug (I'm having Ubuntu 16.04 on VirtualBox), so I can inspect the sass code for ruby-on-rails or php projects? I 'm not entirely sure, but I guess would be a system or a firefox setting, right?
Firebug does not have support for source maps and it is not working anymore once multi-process Firefox is enabled.
Therefore I suggest you use the Firefox DevTools, which support source mapping.