Usually if there is a problem with Firefox's understanding of the proper paper size of a printer, it is caused by Firefox having extracted the wrong paper size settings from Windows when reading the print driver data. Clearing it can involve finding some obscure settings, but here goes:

(1) In a new tab, type or paste about:config in the address bar and press Enter. Click the button promising to be careful.

(2) In the search box above the list, type or paste print and pause while the list is filtered

(3) For each setting that specifically mentions the problem printer, right-click and Reset it. The fastest way is to right-click with the mouse and then press the r key on the keyboard with your other hand.

The next time you use that printer, Firefox should re-fetch the data. Any improvement?

Nope. Same results. Thank you for trying. Any other thoughts?

If you go back to about:config, do the incorrect measurements show up there and can you edit them to the correct ones?

Otherwise, I wonder if there is a metric-to-imperial rounding error somewhere, but since I haven't heard of this before, I don't have an idea of where to check for that.

Also, it only happens on one of my 2 computers. One computer prints pages from Firefox just fine, but the other one will not.

Hope that added detail helps. Thanks again for any assistance you can give.

I got it figured out. Somehow the paper_size_unit was set to 1 which is metric. I changed it to 0 and changed the paper_size_height to 11 and paper_size_width to 8.5. It now works fine.

I have no idea what caused that to happen. Thanks for your input.