CSS\JS Aggregation "Shuts down" Firebug indication
Advagg has a built in feature to bypass aggregation for a single request.  You can append ?advagg=0 to the query string to load the page without aggregation.  I haven't personally used this feature, but it seems it could be useful in your situation.

From the module description:


  Url query string to turn off aggregation for that request. ?advagg=0
  will turn off file aggregation if the user has the "bypass advanced
  aggregation" permission. ?advagg=-1 will completely bypass all of
  Advanced CSS/JS Aggregations modules and submodules.


This would help with your problem of having readable css in firebug, but any changes you make to your sites css would still need a cache clear to take effect.