Does anyone have this issue were firefox becomes slow if left open for a long time. In my case after a couple of weeks rendering becomes slow and when I use youtube for example if is laggy, just trying to change volume taka few second to show the volume bar. It also happens to my laptop at work. I have around 30 tabs open.
Their methodology (and therefore likely yours aswell) is flawed and it was immediately pointed out in that thread too: https://libreddit.bus-hit.me/r/firefox/comments/18gp19l/ram_usage_in_firefox_vs_edge/kd2u2pq/?context=3#kd2u2pq
Measuring the memory "a process" actually "uses" is not trivial.