As I wanted to check what's going on I accepted the upgrade to the alpha, but just in 2 minutes it has forced me to go back to the normal Firebug:
Inspector pane, I select an element, on the right I get the CSS Rules pane. Then I edit the rule for this element, so then Right click to copy the rule and....
It's gone!!!
There's a single (disabled) "copy" option wherever I press, so it seems that it's back to the old days when I have to carefully select the rule or property that I want to copy.
So back to the working version and I'll check back again later.