This seems to work indeed! Thanks for the tip. The target is now: C:\Windows\explorer.exe "G:\Program Files\FirefoxPortable\FirefoxPortable.exe". A little weird, I don't understand why this is necessary but it works.
I have several portable apps, such as FirefoxPortable, installed on a USB drive, that I would like to start from the task bar. I have searched for ways to do this but none of the suggested methods works so far. E.g. dragging a shortcut onto the task bar does not work. Is there another way?