A nice UI feature is in Settings -> General -> Performance.
I still notice, compared to Chrome, that if I use 1 content process, no matter how many tabs I have open, or how many I have closed, the process just grows and grows over time. In contrast, Chrome may use more memory but its size dependably follows the number of tabs open / closed.
Even though FF is recommending 1 Content Process now as default on my low spec machine, I have set it to 2 to increase the likelihood that one of those processes has no tabs, and gets destroyed, along with whatever garbage accumulates.
I think Mozilla could do more research on migrating tabs to processes to minimize memory usage, and accumulated garbage from the runtime that doesn't seem to be collected. Perhaps after a certain threshold (time, size) content processes should be destroyed and tabs migrated to a new one.