Up until yesterday I would've said "Firefox" (because I hoard tabs), but it turns the real answer was "Firefox running as a Snap."
(A failed update screwed up my Snap installation, which finally gave me the kick I needed to quit procrastinating and excise it from my system once and for all. I'm running Firefox installed via apt package from Mozilla's PPA, and now -- with the same number of tabs open -- my system is hovering around 8 GB memory usage, when before it was constantly bouncing off the 32 GB redline.)