If you’d rather stick with the non-debug version for performance reasons or personal preference, I think I have a work-around that will get you up and running.

1) open Firebug in a separate window via the status icon’s context menu (right click).

2) Inspect button should be active, click on it and inspect some element of your open tab.

3) collapse Firebug back into a frame. When you open it again, all elements should be initialized.

Upcoming alpha/beta release should have this fixed up. JJB checked-in R1559 into the 1.4 branch yesterday. Let us know of any other problems via the google code issues page.

Firebug 1.2.1 is not compatible with Firefox 3.1.

That bothers me, but not half-as-much as it probably bothers you, the person trying to debug some web on the latest, fastest, state-of-the-art Firefox (that kicks ass and is really cool and will make you more popular). But don’t fret! It won’t be this way for long. I’m hoping that today or tomorrow we’re going to release Firebug 1.3 on AMO!

Firebug 1.3 is not compatible with Firefox 3.1.

No, I’m not kidding. It sort of works, but there are a number of issues with it. It was never intended to work with Firefox 3.1 and as such, there are some known and probably many more unknown problems.

Let me say what I should have said at the start of this post: If you want to use Firebug with Firefox 3.1, you will need to use Firebug 1.4. Get the latest version from the releases directory and install it up. No compatibility override required. Don’t let the alpha version tag frighten you, it’s really quite decent.

Details about the different versions available on getfirebug.com’s releases page. Worth a read to see all the cool new features that have gone into these upcoming releases.