I have a slow connection, so privacy.partition.network_state = false was a game changer. It prevents firefox to re-download an image (or video, or whatever) when you open it in a new tab (State Partitioning in Mozilla docs).

Warning: the main goal of State Partitioning is to prevent cross-site tracking, so this option will make that easier. In my case it's worth it and I mitigate that by other means, but you should be aware of it.