First, whether it will work on Aptana? Yes, it should.
Now to the main question: The Fireclipse site claims contrary to what the answer you have linked to cites: the FE site claims that it mainly helps in debugging javascript through Firebug and Eclipse consoles (nothing about exporting/saving CSS changes).
Also, I wouldn't really recommend going with this plugin 'coz it hasn't been updated in a while. According to SourceForge (where the source of this project is hosted), Fireclipse was last updated 2008-10-22. Unless, you are still using Firefox 2 and Eclipse 3.2, you should avoid this plugin.
Try some of the other tools listed in that thread, for example, FireDiff sounds promising.