Depends on your work style.
If you want to use Firebug only on one or a few sites:
Use the mini menus on Console, Script, Net tabs and set to Disabled.
(this is the default).
On the one or few, use the enable for site control that comes up on
these panels to enable that site.
If you want to use Firebug a lot, but not on a few sites:
Use the mini menus on Console, Script or Net set to Enabled.
On the few sites, use the mini-menu to set disabled for sites.
In both cases if you have *any* site open that enables Firebug, then
Firebug is active. You have to close the tabs to turn it off.
OR: for any site any time, right click on Firebug and select Suspend.
jjb