Preventing Firefox from replacing "open URL" with "Switch to tab"
Solution 1:
Hit Shift+Enter. Seems to work for me.
Solution 2:
I use the Switch To Tab No More add-on. It removes the "Switch to tab" option entirely, so you don't have to hit Shift every time you try to double-open something.
(That said, I probably would have never installed it if I had known about the Shift+Enter command Oliver suggested.)
Solution 3:
You can also hit Backspace at the front (or edit the url in any way and then undo).
Then the switch to tab
disappears.