I managed to get one of my tabs into a state where I couldn't dismiss the darkened highlight overlay after closing the find toolbar (screenshots attached). I could bring up the toolbar and search again, though. Unfortunately, I wasn't paying attention to how I managed to do this, so I don't have steps to reproduce. If it helps, I noticed "can't access dead object" exceptions in the Browser Console pointing to http://searchfox.org/mozilla-central/rev/bcc9ea6947878ca6378e5b7d6f08c1c090ed9eb7/toolkit/modules/FinderHighlighter.jsm#326. This is with Nightly 51.0a1 (2016-09-12).

How the tab looks with the "double overlays". The overlay went away when I navigated to another page.

Just for comparison.

I believe this to be a dupe of bug 1302178, please feel free to re-open this bug when you encounter it in the next Nightly still!