Restoring closed tabs doesn't work correctly (the restored tab position is incorrect) if https://addons.mozilla.org/en-US/firefox/addon/open-tabs-next-to-current/ is also enabled.
Correction: "Open Tabs Next to Current" extension is responsible for the bug.
It seems that the problem also exists with only open-tabs-next-to-current enabled and this extension disabled. I suggest reporting it to open-tabs-next-to-current.
