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.
Menu -> Developer -> Style Editor, edit a stylesheet (or add / import a new one), then click "save" in the left panel. Done!
More info: https://developer.mozilla.org/en-US/docs/Tools/Style_Editor