It’s 2010, Firefox is going 4.0 soon and it still has no way to tell why it’s sluggish.
Yes, it would be nice if the FF developers could make the browser so fast that it would never become sluggish in the first place. I just fear that this is simply not possible with the current design. Look at Chrome: Every tab runs in it’s own process. So what if one of them is slow? All the other still feel crisp.
If you care, support Bug 505104: Please add a tool to gauge Addon performance.