This extension will let you duplicate the tab with Ctrl-Shift-T, or through a menu you access by right-clicking the tab.
If you don’t want to install an extension, try:
Ctrl-L, Ctrl-C, Ctrl-T, Ctrl-V, return.
to select the Location (URL), copy it, open a new tab, and paste the URL into the new tab’s location, and load it. Lots of keystrokes, but you don’t have to reach for the mouse!
(It’s technically a workaround, since what you’re doing is focusing the address bar and then asking Firefox to open it in a new tab… but hell, it works).