Just curious, how many extensions do you have? I'm trying not to bloat my browser too much with them, but have 3-4 privacy related extensions and a couple of utility ones. Right now I have 22 tabs and Firefox uses 5-6 GB of RAM. Restart decreases RAM usage, but Firefox slowly eats RAM again.

I remember recently I freed 10GB of RAM just by restarting Firefox. I literally looked at Ubuntu's System Monitor. And I keep everything updated.

Tried to play with settings in about:config, made even worse and reset.