Yeah, Firefox is still leaky if you open a lot of tabs then close them, some of the memory isn't freed up. If you check out the page "about:memory" then click "Minimize Memory Usage" it will clear a little of the jam, but you'll notice that the amount of memory
Firefox thinks it's using is much less than they amount
Windows thinks it's using. Hence, it's leaky and you can't do a thing about that except close Firefox.
Even in safe mode, it leaks, so it's not necessarily add-ons doing that. Opening my browser in normal mode used ~470MB, safe mode ~170MB. I have about a half dozen extensions. Opening 40 tabs got that up to 2GB on normal, 1GB in safe mode. Then closing all the tabs and doing "Minimize memory Uage" safe mode still shows a leak of about 150MB, and the normal mode had a leak of about 300MB.
EDIT: there's some more info and slightly helpful tips below. It appears that the problem is actually fragmentation of memory pages. There's a chunk of memory in the about:memory called 'decommitted', which is memory that Firefox freed up but is still allocated to Firefox by the operating system. When Firefox frees up some memory, if there's any data
at all left in that memory page then the entire page cannot be handed back to the operating system. So what happens as you open and close things is that the memory allocated to Firefox fragments. It's an operating system issue really, and it's going to affect
all browsers.
https://slashdot.org/story/06/02/14/2154224/firefox-memory-leak-is-a-featurehttp://kb.mozillazine.org/Browser.sessionhistory.max_total_viewers