What extensions do you have loaded? Done anything else to customize it?
I've been running Firefox for over a year now on my Kubuntu desktop and haven't had that issue. Not with pre-Quantum or post-Quantum Firefox.
Also, some tabs will continually use more and more memory due to poorly-written JavaScript. That's not really Firefox's fault but Chrome seems to be much more aggressive in shutting those sorts of things down (which can be annoying because if you go over that threshold in your own JavaScript it'll stop working and you'll get ambiguous resource errors in the console).
