It costs nothing to keep a tab (you had every intention to revisit later) open, especially when tabs can be auto unloaded to free up memory, either by an addon or the browser itself. Firefox unlike Chrome does not keep squashing more tabs in the tab strip forever, but lets them scroll horizontally out of sight to maintain a readable minimum width for each tab. Over a long enough period of time you can get 2000 open tabs. Modern browsers can handle it.

Not what I asked.

This is not my personal experience. If I let a tab alone and that tab has a looping GIF then Firefox get slower and unresponsive (and eat memory).

You can try it with an imgur link: https://imgur.com/gallery/gom01RZ (sfw, vid/loop of a boa constrictor drinking water from a glass).