I got here looking exactly for this feature, that is, being able to save edited CSS
properties back to the original file (on my local development machine). Unfortunately after searching a lot and not finding anything that suits my needs (OK, there's CSS Updater but you have to register and it's a paid extension...) I gave up on Firefox + Firebug and looked for something similar for Google Chrome. Guess what... I just found this great post that shows a nice way of getting this to work ( built into Chrome - there's no need for additional extensions ):
Change CSS and SAVE on local file system using Chrome Developer Tools
I tried it now and it works great highlighting the changed lines. Just click Save and you're done! :)
Here's a video explaining this and much more: Google I/O 2011: Chrome Dev Tools Reloaded
I hope it helps if it doesn't matter to you changing browser while editing your CSS files. I made the change already for now, but I would really love to have this functionality built into Firebug. :)
[Update 1]
Today I just saw this video: Firefox CSS live edit in Sublimetext (work in progress) Looks promising indeed.
[Update 2]
If you happen to be using Visual Studio 2013 with Web Essentials you'll be able to sync CSS automagically as shown in this video:
Web Essentials: Browser tools integration