Hidden Firefox feature – list of opened tabs with thumbnails?

Today I accidentaly found, that firefox has probably hidden feature. I was always wondering for some feature in the Firefox, which would work similar like Switcher for Windows Aero enabled systems. I mean feature which will offer You list of opened tabs, and You can easily choose one of Your interest via thumbanil or via some letters typed in searchbox.
Today I accidentaly opened this URL and than I left it opened. While I was looking for some exact Tab via shortcut Ctrl+Shift+Tab, something weird happened. On this tab poped up some kind of overlay in firefox, which was offering me list of opened tabs including thumbnails and there is also a searchbox, which is filtering these thumbnails while You are typing some letters inside it. Here is a screenshot of this list:

Than I checked my installed plugins list in my Firefox, but I have no special plugin installed for this feature. Has anyone of You an idea, how did I opened this list of opened tabs? And why is it working only on this page, not for example on Google homepage? And does it work for You too?

UPDATE: I reported this to Firefox bugzilla under id: 566741. So we will see, what they will do or tell us about this.