Sorry, that feature isn't available in Print Preview in Firefox 29, never has been.

Try using { Ctrl + P } to initiate printing when you want to print a "selection".

I hope it's available in the next update, and what is Control P supposed to do. I still can't find the option.

Hold the CONTROL key down, (MAC > 'Command') then the P key - if you have selected text with your cursor you should see a 'selection' choice below "Print Range"

unfortunately, you can't tell what you are going to end up with, so,in many ways it's UN-useful. I always switch to Internet Explorer (which i otherwise would rather not do) which gives you a useful Print Preview where you can adjust the page 'zoom' and margins. It works quite well, in fact !

See also:

• Universal Print: https://addons.mozilla.org/firefox/addon/universal-print/