browser – Firefox tooltip windows are unreadable (black on black) gnome pop-os ubuntu21.04

In Mozilla Firefox, if I hover-over to a tooltip, the tooltip appears to be black-on-black.
No matter what kind of theme I try, it stays like this. I have tried changing the tooltip bg-color in the gtk.css file. And also tried to change the Firefox theme.

What else can I try?

Thanks for any kind of help!