css and javascript aggregation. Can I still edit files?
You can still edit the original files as much as you like, but you won't see the changes reflected in the front end until you clear Drupal's caches.

You can absolutely still use Firebug to inspect your CSS, but keep in mind that the files have been aggregated, so you won't be able to pinpoint the original file and line that a rule comes from.

That's pretty much why it's recommended to keep aggregation off during development, unless of course you're testing the aggregation itself.