Preventing Firefox from replacing "open URL" with "Switch to tab"

Hit Shift+Enter. Seems to work for me.

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.)

You can also hit Backspace at the front (or edit the url in any way and then undo).

Then the switch to tab disappears.