As has been noted a few times, the new activation model in Firebug 1.4 is kind of psychotic. The main problem seems to be that the Firebug developers have disconnected the formerly linked concepts of “open” and “activated” for a few reasons that make sense to the developers, but make no sense to the users. Unless you’ve really internalized how Firebug works under the hood (and there are probably a few dozen people like that in the world), Firebug now seems to randomly choose whether it’s going to be open or closed when you visit a page. Not fun.
The workaround that we’ve found is to force firebug to be active on all pages. You can do this by right clicking the Firebug icon in the bottom right and checking “On for all web pages”. This makes Firebug behave mostly like the old way where the panel would stay open as you browsed around until you closed it. The big caveat is that it only stays closed if you use the firebug icon in the bottom right of your window, not the close panel button.
This will slow your browser down a lot on ajax heavy sites like Pivotal Tracker or Gmail. You can get around that to a certain extent by disabling the Net tab on those domains, which will keep it from displaying every XHR request.
Hopefully this helps, and hopefully the Firebug 1.4 final release has a saner activation model…