The Save as PDF on Firefox defaults to the Operating system based save as PDF dialogue. However, in Opera there is a "Save as PDF..." when right clicking anywhere on a page that gives a WYSIWYG PDF copy of the web page. This has worked for me for a long time, however due to privacy concerns I am moving to Firefox from Opera. This feature is the only reason why I still have Opera on my machine, the Save as PDF produces an EXACT copy of the website, while retaining images and searchable text. Would be amazing to have something like this in Firefox.
Note: there are open source packages (such as https://wkhtmltopdf.org/) however all that I have found is outdated and non-session related (i.e. if I have to login to a website then save a PDF, open source solutions cannot do that at the moment, however Opera produces an exact PDF copy of the webpage).