It's probably because you're tagged onto a post from almost 4 years ago
You can get the firmware from www.hp.com select your country then Support and Drivers, put the model code in and it will take you to the drivers and firmware.
The mention of the virtual usb port is because to send the firmware via USB you need a link to the printer i.e the driver installed or similar, you can use the NET SHARE command to assign virtual links (to the printer alias) but instructions are normally included with the driver on the different methods.
If you have the driver installed then most likely you can plug the printer in via USB, right click on the printer properties and select the DOT4 usb port it's plugged in, the firmware can be sent by a command using the printer share name.
In most cases there is now an easy firmware utility, this lists all the printers it locates by the driver settings, you just click the printer and the easy firmware tool does the hard work, this works great so in most cases I advise our engineers to use this utiltiy whenever it's available for a printer. It saves assigning anything using the net command and avoids the command line bits, its just a couple of mouse clicks.
For some reason the firmware landing page is not coming up this morning but this link will take you to the drivers and firmware page, it's the link at the bottom, if you see the easy firmware udpater try using that first for simplicity.