The preview popup sometimes appears far to the left instead of directly below the tab

[JaneSmith]

If I put my mouse cursor over the very top pixel of a tab, the Tab Scope preview popup appears far to the left, instead of directly below the tab. This is a very consistent and reproducible behaviour for me. Note that the Tab Scope previews appear in the correct location if I put my mouse cursor over any other part of the tab - it's really just the top pixel row that trigger this issue.

This was tested in Firefox 48.0 on Ubuntu 16.04.