Since FF hit the v. 54, it gobbles up memory like there's no tomorrow. The way I used to browse internet before was perfectly fine for older versions (lots of tabs, etc.), but since I updated, I've had FF nearly crash my machine countless times. And I write 'nearly' because I was barely able to kill it via tty2 by the skin of my teeth, but to any non-techie user, the machine looks simply simply brain-dead. Infuriatingly, most of the time, when you feel FF becomes sluggish, closing it triggers another round of mallocs (why? BUT WHY?) that seals the fate of what little free ram you had left, throwing the whole thing on its knees.
When this happens, and there's no free ram left, the load of the machine instantly shoot past 20, and I've seen a peak in top nearing 50 (!!!) ; meanwhile, it doesn't look like FF is swapping, or just a little (500 MB, maybe 1GB, for a ram of 8GB and a swap partition of the same size).
To be sure, I removed FF-KDE and installed vanilla FF, but to no avail. Same problem.
Currently, I consider FF to be unusable. This doesn't please me, because FF has long been a healthy competitor to IE and Chrome, and without it, it might very well have been game over free internet a long time ago.
I don't know if there are ways to keep FF within reasonable borders to avoid locking up the machine, but anyway there are fundamental problems to troubleshoot here.