We* are still generally better at that, but there are many different situations so different users have very different experiences. Please file a bug, preferably with an about:memory report while the browser is using an uncomfortable amount of memory. (You can actually get more info using the Memory tool in devtools, but it's trickier to use and doesn't work after the memory usage has gone too far.) A profile https://profiler.firefox.com/ could also be useful -- it'll record what addons you have, for example, and memory use over time -- but we can figure that out in the bug.

* (I work on Firefox)