Has something changed in Firefox’s architecture? One thought I had is that maybe this has to do with something like page pre-fetching (I think Firefox does that), or something along those lines. I want to sort this out. Chrome seems to be so much better at resource management at the moment.
Update (2018-03-18): Mozilla shared this page to help troubleshoot Firefox memory issues: Firefox uses too much memory (RAM) – How to fix | Firefox Help