All I have to go on is the screen shot you posted back at the beginning of this thread at that moment in time. Go back and look at your own previously posted screen shot.
If you've since closed a number of tabs and/or restarted the machine and Firefox now has different tabs open of course the amount of memory it uses will be different.
I cannot psychically travel through cyberspace to know what task manager is showing on your computer at this moment in time, or any moment in time, other than what there exists a screen shot for. I don't even know what my own machine is using resources wise unless I have Task Manager or a third-party equivalent open.
A PC running Firefox is not "idle" in any meaningful sense of the word. It's running Firefox, that's not idle. Idle is when the OS, and nothing but the OS (and, possibly, antivirus and other background processes essential to safety) are running. As soon as you have an application program running, whether you have focus on it or not, the system is anything but idle.
If your machine is consistently being brought to a crawl, and your memory usage is above 90% during those times, the culprit or culprits as far as what's eating up far more memory than it should will become really obvious when you look at Task Manager. Here's a screen shot of my Task Manager from a few seconds ago, right now Firefox is using between 998 MB up to a bit above 1 GB or so of memory as I watch Task Manager for a few seconds. This sort of fluctuation is perfectly normal as the browser needs a bit more memory and then doesn't need it anymore. I do not have focus on Firefox at the moment nor did I during any of the time period I was watching Task Manager. Firefox is working whether I'm looking at it or not (as is Chrome, Edge, and pretty much any process on the system).
Brian AKA Bri the Tech Guy (website in my user profile) - Windows 10 Home, 64-Bit, Version 1803, Build 17134
. . . the presumption of innocence, while essential in the legal realm, does not mean the elimination of common sense outside it. The willing suspension of disbelief has its limits, or should.
~ Ruth Marcus, November 10, 2017, in Washington Post article, Bannon is right: It’s no coincidence The Post broke the Moore story