I am going to assume you mean your new wireless router cannot find your printer when you try to do the firmware update on your MacBook, so until I know which printer you are referring to, the best advice I can give right now is to print what is called a Network Configuration Page or Wireless Test Report (depending on which printer you have) from within the printer's setup menu to verify your printer is still connected to your wireless network.
If you are trying to do a firmware update, regardless of the printer, make sure the printer and computer have a secure connection with either a USB cord or an Ethernet cord because if you lose your wireless connection or the printer is turned off during a firmware update, then your printer could be broken completely (bricked).
Click the link below for more detailed instructions on how to do a firmware update.
If the link above does not help you, will you please verify the information above so that I may assist you better.