That is puzzling. Firefox should default to using its own internal viewer, but this can be overriden to use an external application. What is the user's setting on the Options page:

See: View PDF files using Firefox’s built-in viewer

If it's critical to have the best fidelity to the original, it's preferable to print from Adobe Reader/Acrobat. If the user is defaulted to Preview in Firefox, there's a download button the user can use to launch the PDF in Adobe:

Ok, now I'm really confused. I use a Mac and it is doing the exact same thing as the Windows user. When I try it in Windows I am getting the same thing as her. I don't get the usual bar across the top. If I open the email PDF attachment (we use the web-based Outlook) in either Chrome or Edge I can click the Print button I get a nice preview screen that lets me make any adjustments but in Firefox I get a very generic Print dialog box. I've included some example screenshots.

My issue is that when she prints by printing the Print button on the window the size will be off and it still includes the Date/time, URL, Pages, etc as if it was printing a webpage when it hasn't done that in the past.

Hi Andrew, it took me a while to get into OWA.

This "Preview" seems to display in a custom viewer on Office 365, and not Firefox's built-in PDF viewer. That part is the same in Firefox and Edge. But when I print in Firefox, the URL is indicated as "about:blank", which suggests that instead of using the built-in PDF Viewer, they are loading their own viewer into a frame for printing. I can't tell why the printout is done that way in Firefox, and because the download URL is extremely convoluted, I don't see any quick workaround to access the true PDF.

Do you recall when this changed?