Reproduced the same issue on Windows 10 x64, using 91.0b8, on the about:welcome page, it seems that the background photo is moving to the right of the page, with both print tab modal (print.tab_modal.enabled = false and with print.tab_modal.enabled=true) see the attachement Portrait+Landscape.zip.
Bug 1676422 Comment 9 Edit History
Note: The actual edited comment in the bug view page will always show the original commenter’s name and original timestamp.
Reproduced the same issue on Windows 10 x64, using Firefox 91.0b8, on the about:welcome page, it seems that the background photo is moving to the right of the page, with both print tab modal (print.tab_modal.enabled = false and with print.tab_modal.enabled=true) see the attachement Portrait+Landscape.zip.