Currently when I look in Firebug I have to guess where the style is coming from.
I know that problem, if you use CSS (not LESS) comments and then compile less without compression then you have the comments in your CSS that can help you to see where the code came from. Inside the normal firebug code inspector view you will not see any comments though. Maybe there are better ways I don't know about.
But its not really a lessphp issue because its just for compiling.
In chrome dev tools(not sure why it doesn't work with FireBug, maybe a bug or my file is too big) you can actually click on the style in the right box and the compiled css will show in the main box. Is there a way we can add a flag to the to the compiler that will insert comments specifying the line number and the file that code was generated from?
I saw something going on here with a plugin called fireless i am gonna check that out. That is exactly what you are looking for.
This feature is called "Source Map" and I can see that is has been discussed here years ago: