New version of firebug, 1.6, has just been released. If you have previous versions, you would receive update message and if you have never installed yet, you can get it from Firebug site or Mozilla add-ons. Here are the best features:

Easier HTML inspection
The node hierarchy is can be scrolled but I think the scroll is not smooth and handy. When the mouse cursor puts on the arrows, the list scroll quickly.

All child can be expanded by right-click on the grandchild node and select “Expand All”. If you do this, you can not collapse them again or I could not find its way.
Finally, the DOCTYPE is always shown.

New console.table() command
The console.table() command allows you to log tabular data to the Console window.

var data = [];  
for (var i = 0; i < 5; i++)
{
    data[i] = [];
    for (var j = 0; j < 5; j++)
    {
        data[i][j] = i * j;
    }
}

console.table(data);

Command line anywhere
Now you can use the command line in any tab, not just in Console. It is useful if you are in DOM or Script tab and want to run a command. Simply by toggle the “Show Command Line” button you can have it.

These features are the best in my view and obviously there are some other new features that I have not mentioned.
Oh, I was forgetting the clearest change, a new icon. When I updated my firebug, I can not find the firebug to start for some minutes, because the icon changed and I was looking up to previous icon!