Hope, I’m right to ask here. I do only see this on Ubuntu Linux, not Windows.
When clicking with middle mouse anywhere else on a page but on a link, then in the same tab the most recently closed tab will be loaded. I find this very annoying as this always happens when middle-clicking slightly next to a link. (i.e. missing it and loading a recently closed tab instead). This drives me crazy.
Does anybody see the same behaviour with Firefox on Linux? I am on Nightly 56 (no addons). To reproduce just go to https://duckduckgo.com/?q=mozilla and middle-click on any of the white space.