I've had this issue since Firefox moved to the new print interface. If the interface appears as the result of the web page I’m currently visiting having an embedded print button, when I print, either via the print interface directly (after clicking on the embedded button in the web page) or from the Firefox menu, the resultant output is nonsense characters. I have two printers, and it’s the same on both.

Hi Stephen, can you give an example of a page that causes this problem? (Preferably one that doesn't require opening a new account.)

If I save the document as a PDF, it’s correctly formatted and readable.

Do you mean setting the "Destination" to "Save to PDF" in the right column next to the print preview, or is this a different button offered by the site that completely bypasses printing?

if I print using the simplified output option, it prints correctly, but obviously doesn't look as good and takes up more pages, but it does work.

That's interesting. Simplified printing usually omits some header and sidebar information. Maybe something in one of those section is causing the issue.

Printing works just fine, unless the web page has embedded code to print the page (or even something else) directly. Basically, when it's instructing the print interface what to print.