But closing tab != closing app.

In ff and chrome tabs are processes. So aside from resources allocated on behalf of that process by other processes not being cleaned up, the OS will cleanup all the memory that tab told the OS to allocated when closed.

Firefox user here, plenty of tabs, Win10 as OS. 3.7 GB before opening the GIF in new tab, 3.8 after opening, 3.7 after closing. Reopening and closing it several times in a row yields the same results, consistently. At least for my setup (Win10 heavily crippled to my own liking) closing tab == closing app in terms of memory gained back.

It should be equal when we are talking about webapps.

It would be fair for a browser to assume that if you’ve just visited one page that you might return soon, and so keep assets in cache for a little while.

Well yes, if done right some browsercaching is fine.

But that could pile up quickly, so I am not sure if and how it is done in the various browsers.