Clicking "Watch Later" from the popup window opens a new window

[LL25255252]

Clicking "Watch Later" from the popup window opens a new window even though I'm already signed-in to youtube.
This new window opening slows down the system and sometimes almost crash my browser.

popup video 0.0.9
Waterfox 56.2.7.1 (64-bit)
Win 7 x64