Do you mean you created a shortcut to firefox.exe and placed it into the Startup folder? Usually a shortcut contains both the command line and the home folder of the program so Windows can understand where to find everything. You could try copying your desktop Firefox shortcut and pasting that into the Startup folder and see whether it works any better.

Windows 10 won't let me place a shortcut to firefox.exe in the Startup folder. Dragging and dropping firefox.exe from the folder c:\Program Files\Mozilla Firefox into the Startup folder wouldn't work either, since Windows assumes I'm going to create a shortcut in the Startup folder, and won't allow it. The only way I could get firefox.exe into the Startup folder was to copy and paste firefox.exe from the c:\Program Files\Mozilla Firefox folder into the Startup folder.

Okay, since that doesn't work, there must be another way. How about a scheduled task? There was a thread a while back: how to start firefox on windows start up.

Setting up a scheduled task worked. Thanks!

Setting up a scheduled task did not work for me. Any pointers would be appreciated.

I tried an uninstall and fresh re-install. It worked.