Firefox has been improving drastically in terms of performance with every release. It's pretty evident in recent months, which is very heartwarming to be honest.
I, however, do to some bad circumstances, have been stuck with a not-so-good laptop (8 GB RAM, a 6th gen processor in AMD A8 7410) and Firefox doesn't run that well on it. This is something that I've observed with Firefox- if you have a decent machine then it will run amazingly fast. However, on lower-end machines, performance can be a struggle AT TIMES.
Any tips on making this browser run at it's best potential on a weak system are appreciated!
This. Switching to Lubuntu from Windows would be like doubling your RAM because Lubuntu is very lightweight. I'd bet your main problem is running out of RAM.