Weird Black Flashes on Firefox

[The Atlae Isles]

I've been using Firefox lately when handling lots of puppets for card farming and R/D, and I notice when a new page loads, it suddenly flashes dark (but not the dark theme) and flickers back to Rift. I've streamed myself doing it, and others see it (so it's not just me).

Tbf I think it's kinda cool and it would be nice to have as a theme, but I guess I just don't like the dark theme.

edit: I used scripts for card farming as well, but I turned them off and the flashes are still there.