You can do a Print Range - Selection when initiating printing through {Ctrl + P} or File > Print, but that option isn't available through the Print Preview window.

There's a Bug filed about adding a feature like that - https://bugzilla.mozilla.org/show_bug.cgi?id=140718 - but considering that report was filed over 8 years ago I doubt if it will ever added.

Dear Sirs:

Please let's get this show on the road. If I need to do an internet search where I will need to print, I won't use Firefox because of this lack of a necessary preview feature.

Thank you.

~ Cliff