Something is F'ed with your system. I have 10 extensions installed, nearly 900 tabs (most of which aren't actually cached) and my Firefox stays open for weeks and it still never uses more than a few GB of RAM.
It's a memory leaking hog on every system I use it on. Windows, Ubuntu, even Android.
Even back to the days of Phoenix, it always had issues with not eventually turning into a leaking process.
The memory usage isn't amazing but it's not terrible. If I go to about:memory and do a report, it seems that nearly all of it is going to Google websites despite being the vast minority of my open tabs. Gmail, Gchat, and Youtube.
So I'm not going to sit here and blame Mozilla because Google websites in particular are hogging a lot of memory.
And this was after nuking the configs and doing an "sudo apt purge firefox && sudo apt install firefox"