I actually installed Firemin[1] and it really helped getting Firefox's memory usage down (while keeping the browser usable!). Some days I wasn't able to play games because Firefox ate up a large chunk of memory, even though I have 32GB installed!

Its pushing Firefox to purge memory more and more, and it managed to reduce memory usage from gigabytes to under 500MB (used, not allocated).

[1] https://www.rizonesoft.com/downloads/firemin/