Use something like ps to list out what is using the RAM
You mention you closed Firefox and it didn’t change but you did not say how you actually measured it.
Start the system up fresh and run this command.
ps faux --sort=-%mem
Or
ps faux --sort=-rss
That will help you track down what is actually using it.
Note: browsers will soak up a ton of memory on modern systems as almost all of them sandbox tabs at this point. If you open a ton of tabs, that will be half your problem.
Someone posted this earlier and it’s a good explanation
https://www.linuxatemyram.com/
Here is an Ug level explanation if you want something shorter:
Disk slow, memory fast (even compared to an SSD)
If memory fast, why no use memory
Some things load from disk often but slow to wait. Load from disk in advance and keep in memory so fast. Now use memory better.
If not enough memory we let go of other thing to run program.
How work? Magic.