Thanks for the reply Sebastian. Yes, this is exactly what I am talking about. :)
Looking at the links,
http://code.google.com/p/fbug/issues/detail?id=4829 in particular, this seems to be an on-going issue that needs addressing but sounds like adding a custom filter to Firebug is difficult to implement.
The best solution, of course, would be better design community awareness. Someone with influence @ Firebug / Firefox could start the ball rolling I guess?? ( I was pleased to see yesterday that the new Bootstrap 3.0 alpha/wip has now dropped the -moz-border bit. Still has -moz-background bits though, which throw up errors. But 'work-in-progress')
Turning off CSS errors will definitely help to reduce the console noise and one can always use the Webkit Inspector for general CSS errors. :) Only thing is, it doesn't help if you are actually trying to use Firebug to look for CSS errors in Firefox ....
Bruce.