... what I have seen the most of is increasing CPU usage if I leave it running all the time. This usually happens after I start get moments of CPU hogging, bogging the entire system down at times.

When I first start FF, CPU usage is typically about 10%. After several hours of leaving it running, I will start to see CPU usage creep up until it gets to as high as 75%. That's when the entire system becomes sluggish, with momentary spikes of 100% CPU usage. #7 on that list of hints given in the quoted section ("Once in a while, close the browser completely and re-open it."), is usually the cure for this.