You don't need a plugin for that. In Firefox Nightly that is already enabled.

You can check this article that shows editing LESS and SASS inside the browser and saving it to the original location on your HD:

https://hacks.mozilla.org/2014/02/live-editing-sass-and-less...

Firefox has lots of awesome devtools but people still think in terms of Firebug and haven't noticed all the new goodies.