I can think of a couple of reasons.

1. Long Firefox uptime
   • If you haven't restarted recently, it has a tendency to slowly use more memory over time.
2. Temporary Downloads in /tmp/mozilla_firefox0
   • /tmp is a tmpfs which means that it actually exists in RAM
   • Firefox stores any download, that you tell it to immediately open with a program, in /tmp
3. Intentionally putting files in /tmp

At any rate, you should see memory usage plummet on a reboot as /tmp will be cleared and processes will start fresh

As for high CPU usage, I'm not seeing anything to indicate this in your screenshots.

I think is firefox's bug. it will kept consuming physical memory on my win 10 laptop too. (recently few main versions, don't remember when it starts)
recommend use chrome instead.