I have a 14gb VM, Sublime Text, VS Code Pycharm, and Hexchat all at open the same time.
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.