Thanks for the answer!
I use Firefox on PC as well, and have no idea what you could be experiencing. Is the card border flashing (they are orange for discard effects and red for trash effects), or the card image itself? Is there an image of an "X" appearing on the cards?
In the options, what choice have you made for "Confirm single trashes and discards" (see attached picture)? If you reverse that choice, does the problem persist?
I'm sorry if I'm asking so many questions, but I still have no idea what is actually happening, let alone how to fix it.