Repeated operation results in 404

I'm an Admin for a Flickr group that requires Admin or Moderator approval for a photo to be admitted to our pool of members' photos. This approval process requires me to view a list of 100 photos, opening them one photo at a time. I use FireFox to open each photo in a new window, review the photo, close the new window, and either accept or deny the photo in the list, at which point that photo is removed from the original list whereupon I review the next. When that list is empty, a new list appears showing the next 100 or so photos. Typically, the number of photos that require approval is around 150 each day, but can be as many as 300 or 400. This had been operating quite well for years, but after the two last updates, the operation has been plagued with 404 errors after reviewing from 90 to 120 photos. As a result, for weeks, I haven't been able to complete this process using FireFox and had to resort to Edge to finish it. I've also seen a similar problem when reviewing photos in Google's Image Search, so this isn't a result of any changes Flickr might have made. FireFox is the problem. BTW, I'm using the current version of FireFox on a Win10 computer, in the unlikely circumstance that it makes any difference.