Firefox normal collection, automatic or triggered in about:memory, will reduce memory used by Firefox about the same as restarting firefox. The latter only SEEMS to reduce usage because it also unloads all tabs which are only reloaded when accessed. If you want to prove that to yourself compare a restart preserving tabs and manually trigger tab unloading in about:unloads
If you are dealing with normal amounts of memory usage and you have less than 16GB you may consider upgrading RAM considering this is as little as $40-60 at this point and well worth the look.
The check before closing multiple tabs didn't get turned off somehow the default changed in 2021 to default unchecked.
Lastly the behavior regarding closing the last window closing Firefox remains unchanged. It does in fact still work that way. If it didn't properly close I would suggest that is a bug or a second window on another workspace. If all you desire is to restart as opposed to actually closing firefox you can trigger a restart in about:profiles this has the benefit of working no matter how many windows you have open.