Activation in Firebug Terminology means enabling its modules, so they start processing the data of a specific website. Firebug supports different activation models. These models were improved over time. The biggest change happened in Firebug 1.4.
Per URL (whitelist)
The default behavior is to enable Firebug per URL. This means that if you open Firebug via
F12 or by clicking its Start Button being at a website, the next time you visit the same website the Firebug window will be opened again.
It thereby follows the same origin policy. (Note that there was one exception in regard of subdomains not being treated differently. This was fixed in Firebug 1.11.)
Firebug is activated for http://getfirebug.com. When you go to http://getfirebug.com/wiki/index.php/Main_Page it will be activated as well. Also if you go to http://blog.getfirebug.com the Firebug window will be opened. But if you go to http://mozilla.org, it won't be activated.
There's a preference Activate Same Origin URLs to control if the activation should follow the same origin policy. If this option is disabled, Firebug will disregard the origin when deciding to activate or not.
Firebug will always be activated. This means Firebug will be activated regardless of which page you visit. This behavior can be enabled via Firebug Start Button Menu > On for All Web Pages.
Having this option enabled also hides options to deactivate Firebug.
Besides the general activation of Firebug there are also some panels, which allow to enable/disable them. The reason for disabling single panels is to avoid wasting system resources for things you don't need. E.g. as a web designer you probably won't need the Net Panel functionality.
The panel activation model currently only supports an on/off state. I.e. if you enable the Script Panel, it will subsequently be enabled for all sites Firebug is activated for.
Currently Firebug has three panels that support activation:
Also there are some extensions, which allow disabling their panels.