Hey all, just found this thread and I thought I'd contribute some findings that may or may not help... I noticed the large memory footprint and alongside I noticed that there was a Macy's ad, owned by the Orion process that was in control of the MacOS media controls widget (there were no tabs playing media at the time).
I think maybe the content blocker is removing elements after the DOM parser has generated media elements for the content which is subsequently removed by the blocker. I suspect the media controls handle is being leaked for every video element destroyed by uBlock Origin, which if the site is tricky (using JS to inject ads) could lead to an infinite loop of spawn/kill/leak cycles.
Twitch.tv in particular is egregious about generating a lot of memory thrash via this bug.