background-attachment:fixed; doesn’t work when any ‘transform’ is applied and that’s a bug in firefox and it is not yet fixed. Reference: https://bugzilla.mozilla.org/show_bug.cgi?id=1292499
This has been a very common unsolved mystery. Recently I had the same problem, and ‘-webkit-backface-visibility: hidden’, proved to be less than useless (on my ‘fixed’ attached background), since the background just disappeared when it was set. (Additional Info: the reason is that when the background is set as fixed, it is almost similar to … Read more
Unfortunately this is simply an artifact of how fixed positioning works in CSS and there is no way around it in pure CSS – you have to use Javascript. The reason this happens is due to the combination of background-attachment: fixed and background-size: cover. When you specify background-attachment: fixed it essentially causes the background-image to … Read more