STR:

1. In a fresh profile, load the attached testcase.
2. Ctrl+P
3. Ensure that "Save to PDF" is the selected print target.
4. Click "Print using System Dialog"
5. In the system dialog that appears, click the "Page Setup" tab.

ACTUAL RESULTS:
That tab in the system print dialog shows that "Orientation: Portrait" has been chosen. (And if you proceed to print, you'll get a portrait-oriented sheet with this tiny landscape-oriented 4x3 card centered at the top of it.)

EXPECTED RESULTS:
The orientation should be shown as Landscape.

(This is along the lines of what was described in bug 1836028 comment 8, though I'm not yet sure if that comment is about the same underlying bug or something completely different.)

Here's a screencast (taken in another fresh profile) showing that I do get "expected results" if in step 3 of the STR I change the Destination in Firefox's print dialog to my physical printer (before I click out to the system dialog).

Interestingly, the displayed paper size differs between those two screencasts; in comment 1 it's "US Letter", whereas in comment 2 it's "na_letter".

Thanks for looking into it. I opened Bug 1838989 a few days ago with a few more details. Perhaps it's related.