Hello! Attaching the regression range with the print preview loaded as expected after the ok button is clicked on the warning message.
Last good revision: 59db3b0781908d9e2145477ff8dc5a85729e25bf (2016-09-30)
First bad revision: 87cd291d2db621da6b3eb1057027cc0725b6eb1d (2016-10-01)
on Firefox 52.0a1 (2016-09-30) when clicking print before the PDF loads: print preview preparing message is opened in a new tab and clicking ok on the warning message from the original tab with the PDF, opens print preview with the pdf loaded as expected. This was taken as a good build.
on Nightly 52.0a1 (2016-10-01) (64-bit) when clicking print before the PDF loads: a new tab is opened with Print preview preparing message, and going back to original tab and clicking ok on warning message loads only a white page from the PDF in print preview like in comment 0. This was taken as a bad build