I am using FF 51.0.1 (32-bit) and I have this issue with Firebug. When Firebug is not 'off', the window 'quick info/computed style' remains on the screen and is hidden. I barely can't click on the area where it is placed.
https://drive.google.com/file/d/0B7joCxHiM5SWa1hkeFJwOFJ5dVU/view
That's obviously a bug in Firebug, though note that Firebug's development is officially discontinued. Reasons for this can be read at the related Mozilla Hacks block post.
It advises to switch to the Firefox DevTools, though the DevTools don't have that feature yet and I couldn't find a request for it. So, you may create one and mark it as blocker of bug 991806.