Hello,
on Firefox Nightly, the popups from the UI are totally unusable.
Examples : delete user, delete repo…
The popup opens and then flickers, or disappears on mouse move…
I have to user Chrome for certain tasks.
Can you still reproduce this? I’m on Firefox 59 and I can’t. If so, can you make a video showing what happens? I don’t think I quite get what you mean… 
It was only on FF Nightly, and hard to explain, i should have made a video 
I just tested on latest FF Nightly 61.0a1 and i don’t reproduce anymore.
Nightly isn’t stable, so i think you can close this issue.
I see. Thank you for your report nonetheless!