Try resetting the ZOOM in Firefox for that domain, while you are viewing one of those pages.

https://support.mozilla.org/en-US/kb/font-size-and-zoom-increase-size-of-web-pages

It is not a Zoom issue. I have already followed the troubleshooting guide from Mozilla. This issue is present on every PC I have tested. Both Windows 7 and Windows 10. The issue apparently is related to outlook online. The Zoom settings are normal and I have reset the printer settings back to default in about:config. Based on my investigation, it looks like Firefox is unable to separate pages in the email thread and tries to shrink-to-fit the entire email. Thanks

It is also worth noting that it only happens on long email threads that are being printed. A single email prints just fine

Hi gad2, this is a nightmare!

So the first problem is that some element do not "wrap" and therefore Firefox has to shrink them down to fit entire lines within the margins. There are hundreds of style rules in the document that specify

white-space: nowrap

and those probably are not helping. We can override all the white-space rules in one go using this (it simply tells Firefox to do what it normally does with white-space):

@media print {
  /* Override "nowrap"  when printing */
  * {
    white-space: unset !important;
  }
}

You can use an extension like Stylus (https://addons.mozilla.org/firefox/addon/styl-us/) to apply rules like that to sites automatically.

Now this doesn't solve all our problems because the content cuts off. There are certain style rules that Firefox treats as creating unbreakable blocks. Instead of being split over multiple pages, the content just runs off the bottom of the page. Frustrating. I have an add-on that lets you inject style changes into pages to improve the page breaking behavior, but it doesn't cover the white-space problem.

https://addons.mozilla.org/firefox/addon/printable-the-print-doctor/

(This is giving me a headache.)

I think if you have a chance to complain to Microsoft about this, it might be useful.

I will apply this coming week and post results. Thanks!

The message header area is also a disaster. This helps somewhat:

@media print {
    /* Override "nowrap"  when printing */
    body, div, p, span {
        white-space: normal !important;
    }
    pre {
        white-space: pre-wrap !important;
    }
    /* Widen subject line */
    div.showOnPrint > div:nth-of-type(1) {
        width: 100% !important;
    }
    /* Fix text color and alignment of headers */
    div.showOnPrint > div:nth-of-type(3) > div:nth-of-type(1) span {
        color: #000 !important;
        display: inline !important;
    }
    div.showOnPrint > div:nth-of-type(3) > div:nth-of-type(1) div {
        min-width: 75% !important;
        float: none !important;
        padding: 0px !important;
        margin: 0px !important;
        border: none !important;
    }
}

I saved it here: https://userstyles.org/styles/173921/clean-up-office365-owa-message-printout

In the bug tracking system (1565682), someone mentioned a workaround:

"Since Firefox V 68.0.1 we now successfully print our mail using the "NEW Outlook trial/beta"."

Visually, you'll see this difference:

Note: in order to get the correct part of the overlay to print, you have to use the print link provided in the page, just using Ctrl+p doesn't work.

Thanks so much for your assistance.