I did not know until I just discovered that if I try to drag a tab from Firefox onto Chrome, it opens that page in Chrome. (The existing Firefox page stays where it was).
The dropped Firefox tab opens a new tab if you drop it in Chrome's top tabs bar. It replaces an existing page if you drop it on the address bar.
The reverse (dropping Chrome on Firefox) is not an option.
Chrome is eerily a bit T-1000-ish - morphing to intent.