Is there a firefox plugin that lets me edit CSS in the developer tools AND lets me save the edits back to the actual CSS stylesheet on my machine? I'd love to see that functionality.
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