CSS-X-Fire
I'm surprised that it still not listed to this question, but probably because is new and the author didn't have time to promote it yet.
It is called CSS-X-Fire and it is a plugin for JetBrains series of IDEs : IntelliJ IDEA, PHPWebStorm, PyCharm, WebStorm, RubyMine.
How it works:
You install one of these IDEs and configure the deployment (supports FTP and SCP). This will allow you to stay in sync with the server.
After this you install this plugin. When it starts it will ask tell you that he will install a plugin for Firefox, in order to do the integration between Firebug and the IDE. If it fails to install the plugin, just use the drag-n-drop technique to install it.
Once installed it will track all your changes from Firebug and you will be able to apply them with a simple click inside de IDE.
FireFile
FireFile is an alternative that requires you to add one small php file to the server side in order to be able to upload the modified css.
