best way to screencapture stills from YouTube videos?

[Aryeh Friedman]

I am using xfce4 on ff 25.0:

Shockwave Flash

File: npwrapper.libflashplayer.so
Path: /usr/home/aryeh/.mozilla/plugins/npwrapper.libflashplayer.so
Version:
State: Enabled
Shockwave Flash 11.2 r202

Everytime I try the screen capture util for xfce4 I get a black screen
where the video should be.... what I am attempting to do is capture
the still at 3:07 in https://www.youtube.com/watch?v=2IkBRRAA9hM
(attempting to get my dad to identify all the people in the photo ;-))


-- 
Aryeh M. Friedman, Lead Developer, http://www.PetiteCloud.org