Jumped to 950 MB, settled at ~800 MB. Deferred loading seems to do a lot, so I could imagine that different usage patterns make a big difference, but that would need a detailed look into how Firefox and Chrome manage memory for "stale" tabs. If I remember right Mozilla recently added improvements there, getting rid of their memory more aggressively?

So, under actual usage (not ctrl-tabbing around to aggressively force-load things) it is 950 MB vs 5.3 GB. (I imagine for a comparable number of tabs?)

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EDIT in response to your reply:

Please do! I agree that logging would be interesting.

Do you know, does Firefox defer/discard tabs again if they do fully load but you don't use them for a long time? Or are the only deferred tabs, ones that you never looked at this session?

I don't think you can count a freshly started browser that has only loaded 7 tabs (one per window) as "actual usage". I'd guess tabbing around triggered <50 tabs, that doesn't seem like a totally unrealistic working set, at least not when I'm actively researching something.

Maybe I should set up logging for this, might be interesting.

Please do! I agree that logging would be interesting.

Do you know, does Firefox defer/discard tabs again if they do fully load but you don't use them for a long time? Or are the only deferred tabs, ones that you never looked at this session?