Occasionally, Firefox will not recognize the operating system's default printer, but will have its own independent default printer.
To change the default printer Firefox selects, enter "about:config" in the address bar (without the quotes). You might be presented with a warning message; bypass this. Then, select "print.print_printer" under the column "preference name" (using the "filter" field, you can search for preference names instead of scrolling through all of them). Confirm this entry is of "string" type. Double-click on this entry, and enter the full name of the printer here as the "value" for the preference (\\goprint.its.carleton.edu\CMC119-X7120, for example).
If "print.print_printer" is not defined, you can create a "new" string by right-clicking within the window, and selecting New -> String. Enter print.print_printer as the preference name, and enter the printer name as its value again.