I have a dual monitor setup where my laptops built-in screen sits at the bottom and I have another screen sitting at the top. When Firefox is maximized in the bottom screen, and I open Inspect, it opens up in the top screen, which is my physical monitor.
Ehsan: By inspect do you mean the highlighter or the style inspector?
Considering the Style Inspector hasn't landed yet, probably the HTML panel.
The HTML Inspector, which is available from Tools > Web Developer > Inspect.
Given our plans for the UI (the stuff that's floating around is going to get docked), this bug should automatically resolve once we've finished. Keeping it open to track just in case plans change, though.
The HTML panel was moved in-window in bug 689946.