You can also check if the print preview is okay. You may also want to try toggling the options for fonts (if present) like embedding via Firefox File (Alt + F) > Print > Properties for the PDF printer.
Re: Firefox prints all webpages as images while printing as pdf.
If the printer (PDF printer settings) has an option to Embed fonts you can try selecting that. Another thing you could try is to print a sample from notepad. If it's okay you can try that font type for Serif in Firefox Tools (Alt + T) > Options > Content > Advanced (beside Fonts & Colors) > while deselecting Allow pages to choose their own fonts, ... and check.