What does the Finder or browser icon marked "From Mac" in my Dock mean?
Sometimes this appears in my Dock:
I've seen it with Finder and Chrome icons as well. When I click on it, it brings the app to the foreground. Sometimes, it says "Handover failed" or something similar. What does this icon represent and what is supposed to happen when I click it? This is on macOS 10.14.6.
That is telling you there is a webpage (most likely) that was sent from another iDevice to this one.
When you click on it, it will open Firefox with the shared item.