Matthaus,
Welcome to the forums! As D said in this thread:
http://discussions.apple.com/message.jspa?messageID=9165237#9165237
"You can print in grayscale from an HP print dialog box. I don't have the same printer but the options should be similar. Click the Triangle beside the Printer dropdown menu (if the printer options are not showing.) This should expand the dialog. There should be a dropdown that says Firefox, click it and select Paper Type/Quality. The first tab should be Paper. Select the dropdown for Color and select Grayscale.
If the above isn't the same as yours, there still should be the preference somewhere in the Printer Options. Hopefully, that will work for you.
D. "
Or You could always: Go to print like you normally would but instead of hitting print click on the (PDF) button near the bottom. From the list it show select (Save as PDF...). Now I would save it to the desktop for easier finding. From there open it in (Preview) and do a quick (Save As). In the little window that opens you will see a (Quartz Filter:) select (Gray Tone) from the drop down list. Click save and then print the gray toned PDF you just made.
Hope that helps,
Weston