My experience of using a weak machine (HP8560p, 12 years old) so I guess it's okay:

• uBlock Origin + Auto Tab Discard to get minimum amount of RAM as possible

• about:config: Turn off almost everything, telemetries, Firefox View using only 0100: STARTUP to 0600: BLOCK IMPLICIT OUTBOUND... You may want to check BetterFox and use only FastFox: https://github.com/yokoffing/Betterfox/blob/master/Fastfox.js

• From my experience, for slow computer, you should raise nglayout.initialpaint.delay and nglayout.initialpaint.delay_in_oopif to 2000 to prevent your Firefox from re-render webpages too often.

• And this is extreme, DON'T DO unless you want more memory: Disable Fission, I won't leave a guide here because you should research about this and acknowledge what you're doing.