Rainy Day Spider Solitaire a card will freeze

I'm having this problem, too.  I just noticed it today because of the challenge; I haven't played it in quite a while.

What I did:

• played the game normally at first
• clicked on cards bottom right to deal more cards out
• tried to drag a card that was just dealt

What I expected:

• the card to highlight and move

What actually happened:

• the card above it was highlighted instead
• I tried moving other cards and could move the card above the dead one and place cards on it, too, though they would disappear behind the dead one.
• Generally, it's as if the software has completely forgotten there's a card there except the picture remains.

Other things you should know:

• I tried the same game twice; in both cases, dead cards appeared after immediately or soon after dealing more cards onto the initial set.
• Taking a screenshot wouldn't help because you can't tell a card is dead from that, in most cases.
• I have not found a workaround.
• I'm on a Mac Pro running MacOS 10.14.6
• I'm using Firefox 82.0.2 (64-bit).