Firefox will not print to my HP PSC750 inkjet printer but prints with no problem to the monochrome Brother HL-5240. Other browsers print to both printers with no problem

Firefox will not print to my HP PSC750 inkjet printer but prints with no problem to the monochrome Brother HL-5240. Other browsers print to both printers with no problem. I must print out filled-in webforms and must use the inkjet printer for that. However, Firefox presents me with a dialog box that says "An unknown error has occurred while printing", even when nothing at all has even started to print out. When I next try to print the same screen content to the monochrome laser printer, the entire contents of the form showing on the web page will print out as a single block of text - all run together and utterly unintelligible. How can I make Firefox print normally and legibly to my HP inkjet printer, the same as other browsers? == This happened == A few times a week