Hi

Save as .pdf is currently not available in Firefox for Android, but may be included in a future update. In the meantime, if you hold down the volume "-" and the power button on your device, Android will take a screenshot and save it to your pictures.

Depending on your device, you may have something like the following:

menu > Share > Samsung Print Service Plugin

Firefox passes the URL of the page to the print service, which has a Save to PDF option. This works for public pages, but doesn't work for content that can only be viewed when you are logged in.

It would be more convenient if this were built-in to Firefox, so hopefully that (or an add-on) will be enabled in the future.