do you have swap set up?
also, don't believe firefox is total blame but you can dial in sharper than gnome system settings with
htop
lsof
iotop, sysstat, and dstat are fasincating input/output & sys monitoring tools, too. A little over my head but I get the picture of the processes that consume and what's consumed and how🙂
I digress: but I fear I have too many cli programs already