Depending on the screen size and e.g. the set "reader width" pages might not have filled out the full img elements size due to "object-fit contain".
This was caused due to the image wrappers flex container "align-items center" which slightly reduced the images width to some ".xy" number (e.g. instead of wrappers width 258px, the images were something like 257.83px)
on firefox images are decoded async which causes a "flicker/blinking" when they're getting visible for the first time
this is an issue especially in the reader because pages that should not be shown are rendered but
not displayed, which then causes this issue once they get displayed
schroda