Firefox doesn't have a one-shot feature to capture a page to PDF. It's not far fetched to imagine that one could be created because this works to paginate the image:

(1) Resize the window to avoid wasted margin space, call up the Screenshot feature, Download

(2) Click the Downloads button, then click the folder next to the downloaded image name

(3) Drag the image from Windows File Explorer onto the open tab to replace the page with the captured image

(4) Call up print, choose Save to PDF as the destination, then adjust the scaling to your taste and Print

Making that a one-shot process would be awesome.

Mozilla has a product suggestion site at https://connect.mozilla.org/. If you have time, I think it would be a good idea to post this request there to get into the current feedback stream. Use the "Ideas" section of the site (at https://connect.mozilla.org/t5/ideas/idb-p/ideas).

As a side note, on some sites you can skip the screenshot and just print to PDF, but this assumes their style sheets do not wreck the layout when you print.