Missing the reason

First detailing the memory issue I see: For me it's generally youtube, but I expect the same happens on other "infinite scroll" sites, particularly image (thumbnails for youtube) -heavy ones. My particular issue may be exacerbated by extensions, likely either HoverZoom+ when actively hovering over many thumbnails over time (thus assuming the memory for those is leaked), or, someone else's theory I read, that uBlockOrigin's blocked connections somehow don't fully close and thus leak memory over hours of blocking youtube ads, etc.

Regardless, using a single tab for hours on youtube, either searching (with perhaps a thousand hover-zoomed thumbnails over several hours), or playing multiple videos (tho fewer longer videos doesn't seem to leak as much as many short ones, could be from either many more blocked ads or many more thumbnails loaded, tho with fewer zoomed than the first scenario) eventually uses multiple gigabytes of RAM, even sending me gigs deep into swap on a 16 gig RAM system if I let it go long enough. (Tho I've never had it actually crash, likely because I have an equal 16-gig swap partition on each of four SSDs (so I can stripe them using equal swap-priority values for all four), so 64 gig swap in addition to 16 gig RAM, and I've never let it get beyond single-digits gig into swap.)

In the search scenario, if I middle-click to launch the actual videos in new tabs, then close those tabs in a relatively shorter time to go back and search further from the main search tab, the newer tabs don't seem to accumulate as much and closing them doesn't seem to release much either -- the big memory usage is all in the main search tab that has stayed open. And simply refreshing the page doesn't reclaim the lost memory, nor does deleting cache (which is on tmpfs so does use some memory), so the culprit isn't the tmpfs cache. And FWIW, forcing garbage collection from about:memory doesn't help either -- whatever's leaking, firefox doesn't consider it garbage to be collected!

*BUT*, either duplicating the tab (along with its session history) and closing the old one, or (with another tab open, say just open a new-tab to its default page, or one of the separate-tabbed video pages, so the entire browser doesn't close), closing the offending tab and immediately doing a reopen-closed-tab, keeps the tab's session history, but releases all that leaked memory, sometimes double-digit gigs of it!

So I don't close/kill the entire browser, I simply ensure at least one additional firefox tab is open so the browser itself doesn't close, then either duplicate the leaking tab and close the old one, or close and reopen the leaker. That gets my RAM back without killing the full browser.

Perhaps you'll find that useful as well, tho admittedly, in some cases it's likely less hassle to just do the kill and restart thing. (Much like after upgrading various in-use libraries and apps, while reverting to a text-mode login and running something to tell you what's still using deleted/stale files and restarting it, plus possibly having systemd reload itself too if it's on the list, can usually avoid a full reboot, it's often just easier to just do the reboot and get everything fresh all at once, rather than doing it one app at a time.)